Honestly each Street fighter is very different, but the difference between each version of a game are not mindbendingly different. I belive that ST is 2 but faster and with balancing updates (among other things) but at its heart it is still street fighter 2.
Some people like Alpha 2 more than Alpha 3, but they're both still Alpha, which is not like 2, and more like 3 but still not like SF2 or SF3.
Within these versions there are differences, like apparently Street fighter 3's first and second versions were not very good, but 3rd Strike is on a pedestal of the FGC. But for the most part they're mostly some updates here and there, often times making the game faster or more complex, and new characters.
For casual fighting game fans like me, the differences in the versions are not going to affect the game by a huge amount, though there are things here and there that will definitely be evident.