I feel the same way about various FF games:
1 & 2 - both felt primitive, ineffective hits!, twins (fighter & frioniel)
3 & 4 - a notch up in gameplay, both have this similar feeling I can't explain
5 & 6 - 6 is like a refined version of 5, but without the job system, significant diff in graphics from 4, optimum equip, starting in 5 I felt more in control of the game, etc.
7 & 8 - the "modern" ones, great stories, junctioning reminds me of materia
Past 8 I don't seem to pair them up.