I played some Street Fighter II on Pc Engine yesterday.
I think I am pretty decent at the SNES game (or Super Famicom) so I can tell what the difference is between the two games. And I will say: not much. The graphics is a little worse on the pc engine version but not in any way that effects game play. The sound and music are more synthetic. Game play is almost identical though. I played it to the end on the second highest difficulty level.
One small difference I noticed is that you can kick the Spain Boss (Vega?) as he jumps on the fence, and that makes him easier. Another difference is that you get dizzy more easily but it applies to the opponent too.
It can be a little difficult with the standard two or three button pc engine controller. You change between kick and punch with the Run button. It actually works reasonably, but it makes it a little slower to make a fireball with Ryu and I had some problem with the final boss (Bison?) because of that. They did release a 6 button Pc Engine controller just for this game, but it is relatively uncommon.
Too bad I didn't get to see the credits but maybe that is only on the Turbo version on SNES?
I also played a little of the Saturn port.