Got another observation that could help.

Fi - especially blocked with Se, is much more radical and sharp than Si which comes blocked with Fe and Te. It sorts things out based on sentiment polarization. Or on how good an idea is when it's blocked with Ne.

Si sorts stuff out due to what is comfortable to a person based on efficiency or emotive actions. It can be very lenient: That's why Si egos are paired with fantastical Ne ego brats wink wink.

Summed up: Si is more mellow, especially in Alpha. Fi - a lot less, especially in Gamma.