The first of each iteration has never been its best
SF2 turbo >SF2 - World Warrior
SF3 3rd Strike > SF 3 - New Generation
I am sure it will be the same in the case of SF4.
I say let Capcom refine the game til people settle into what is felt to be the best edition, its what they do best
If your a person who is not interested in buying them all, wait till you can see which becomes the crowd favourite (I highly doubt it will stay as SF4)
Just my 2 cents