(I know this goes without saying, but I'm paranoid and would like to ask those who are privy to the details of my personal life to not divulge any information beyond what I bring up myself in this topic.)
I've amassed a fair bit of personal experience with both of these intertype relations. That is, I've been in relationships with two IEIs and two EIEs -- one of each subtype -- and I've developed what I think are some neat insights.
My previous relationship was with an Fe EIE, and right now (and hopefully permanently) I am in a relationship with an Fe IEI. Both are compatible subtypes for Ti LSIs, such as myself, so I think this is the perfect comparison, and it will be my primary focus. So perhaps instead you might think of this as subtype-aligned Activity v subtype-aligned Duality.
One of the most stand-out features for me was the initial attraction I had for both women. With the EIE, I would describe the attraction as cautiously optimistic. A sense of trepidation mixed with a feeling of intrigue and thrill. "She'll be the death of me" mixed with "She's awesome and fun."
With the IEI, there were no downsides. Rather, there's an immediate, powerful, mutual attraction unaccompanied by any negatives. "She's awesome and fun" mixed with "She's awesome and fun." So I think I can tentatively say that there is a much stronger initial attraction to your Activity partner.
A key, oft-noted aspect of Duality is the way in which both partners naturally, unconsciously facilitate the psychological betterment of one another. The key words here are naturally and unconsciously. Perhaps an even better word would be effortlessly.
As an example of what I mean, one time at school I encountered the EIE waiting in the snail-paced line to the financial aid office. I went over to speak to her, and she mentioned that she was going to skip a class in order to once and for all take care of the bureaucratic horror that is college financial aid. She seemed visibly perturbed by this, not at all confident that she was making a sound decision (Te Role).
My response was firm and resolute. Instead of listing out practical reasons for staying or going, I said, "Class should take precedence over something that can be done at anytime (Ti). You should go to class and come back later (Se)." Note the hard-line nature of the first statement, in which I take on the role of Ti for her, not expecting any feedback. Then note the Se that serves a kind of firm-but-gentle poker. "Here's your Dual-Seeking, now go fulfill your Hidden Agenda." She instantaneously appeared to lighten up, saying, "That's a good idea," and off to class she went.
With Activity, this kind of help can occur, but it requires conscious effort. The IEI I am with was complaining about having a creepy boss who occasionally makes inappropriate physical and verbal gestures toward herself the other female employees. For example, he'll sometimes lightly poke female employees or massage their shoulders while they are working. That's obviously totally unacceptable, and my typical advice would be to make a Ti statement about why that's bad, then offer an Se nudge advising her to report him. However, I knew that she would be very uncomfortable doing this, and that it would be better if I took on the role of Se for her, in a kind of "Fuck this!" manner, then using Ti to clarify and explain my actions. So I wrote down a list of all the things he has said and done, as well as the names of the girls who have been harassed. I would actually call today (in fact, I just now sent her a text informing her of my intent to do so), but she objected on the grounds that they're too busy and that she doesn't want the day devolving into chaos. I'm only going to hold off so long though; you don't touch my fucking girlfriend you foul specimen of human trash. Lucky I don't beat your ass. ajfgkdshfdhfsd
Sorry, off on a tangent. But the point is that she loves having someone take on this Se role for her, and all it takes is this kind of leaping outward for me, her Activity partner, to do so pretty effectively. The subtype alignment makes it easier too, since her Se expectations are much more in like with my Se output. Even the way I pursued her was Se-oriented. I just decided to seduce her one day, and that's what I did. No mystery as to why it worked.
As far as the Super Id help that I receive, it's actually very similar with both. With both, I get lots of happy, inclusive Fe output coupled with a subdued, more playful Ni that serves to encourage instead of forebode. This subtype alignment creates an asymmetrical dynamic that makes it easy for the IEI to help in these areas. Recently I was anxious about something, and it put me into this dark, sullen mood. My IEI noticed this and said, "I love you. Stop worrying so much, everything is going to be fine." That's exactly what I needed to hear and it made me feel better right away. I do worry too much and it's silly.
Okay, so just to reiterate, the point here is that aligning subtypes create a kind of mini Duality that can still help while having all the fun of Activity. It's more awkward, but it doesn't feel like a chore. It just seems like happily going out of your way to help, and it's deeply appreciated by the recipient. "It makes me happy knowing that you'll defend me" isn't something an IEI says to someone who is pocketing their Se.
This segment went on way too long lmao.
Duality is like .5mg xanax and 40mg adderall taken at once. Invigorating yet relaxing. A balance of two drugs that were made for each other. The adderall makes things fun, and the xanax eases away anxieties and insecurities so that you feel comfortable even talking about your weaknesses.
Activity is like 80mg adderall followed three hours later by 5mg ambien. VERY invigorating, but the come down is more dramatic. Fun as hell, but you both need to pass out and get some sleep afterward. I think this holds true especially for introverts. Several descriptions claim that both partners need to be alone, but I find that it's more that both partners just need to take a nap, but can do so in the same bed. You don't mention weaknesses because neither of you perceives any in the other.
I think of Duality as two partners staying up all night in a castle library, in big chairs by the fireside, talking and losing track of time, intimate, like a little pocket world, delighted with each other. Relaxed yet stimulated. I see Activity as Belle and the Beast dancing in the library during the day, all colors and music and energy that will inevitably be followed by passing out in a feather bed. There is overlap here, of course, but that is how I'd articulate the far end of either spectrum. The thing about the subtype alignment is that it shortens the span between either extreme. The Dual is more "active" and the Activity is more "dual-ish," though each relation is still closer to its respective side.
In this area, Activity wins out by quite a bit. There is very little, if any, reason to stay mad at your Activity partner. You don't even quarrel. It feels solid, yet never boring.
Duality is a love-hate sort of thing. You love them but they drive you crazy. Unfortunately, this can easily lead to breaking up. I'd wager there are far more Duality divorces than Activity.
Closing, the reason I wanted to write this because, with my grasp of Socionics, I can see that all these little problems described in Activity relations aren't actually big deals at all. Like I said earlier, I've been in relationships with both an EIE and an IEI of aligning subtypes, there are noticeable differences, but it doesn't feel in any way worse or less fulfilling. And the feeling is mutual, us both having had Dual partners in the past and feeling the same way now. With a little effort, you can get some missing Duality features while retaining the benefits of Activity.
I know I kinda bashed Duality in this topic, and made it sound like Activity is better, but that's not what I think. While I do maintain that Activity trumps Duality as far as initial attraction and relationship stability are concerned, Duality is obviously better-aligned as far as input-output. However, with a decent grasp of Socionics, as well as the presence of subtype alignment, it's pretty fucking easy to make Activity relations just as fun and happy.
I'd rate them 50-50.