The obvious difference between LSIs and LIIs is their respective use of Ne and Se. Let's first focus on Ne.
In my mind, Ne is a function that is highly capable of generalizing and being aware of complexity. Ne-leading types tend to be most aware of "grey-areas" and most tolerant of ambiguity. In general, types with Ne in their ego are also capable of theorizing in a skilled manner, because theorizing involves generalizing and being tolerant of grey areas - theorists can never be certain that the theories they've developed are necessarily true.
In this sense, LSIs are generally less capable than LIIs at all of the above. Because they are less aware of and tolerant of ambiguity, they are more likely to be categorical, stubborn, and they often see things in black and white. Even though she's not LSI, my mom is a classic example of an Ne-PoLR. She sometimes makes obvious errors in logic, and, no matter how hard you try to correct her, she won't listen.
This isn't to say that LIIs are superior to LSIs (or ESIs). Each type has their own advantages and disadvantages. While LIIs might be better-suited with less well-defined and more general information, LSIs, due to strong Se, are well-suited to areas in which the rules are extremely clear, and in applying their logic to specific situations.
Perhaps the best career for an LII would be in academia. This is because academia allows an LII to apply their logic to more abstract and less well-defined types of information.
Perhaps a good activity for LSIs would be chess. I'm not an expert at chess, but it seems to involve applying logic and dealing with tangible objects, specific situations, and well-defined rules.