Vice City Stories for PS2 was released in 2007 while Vice City was released in 2002, so there probably are some improvements in graphics (just a guess).
I really liked the original Vice City and I've only played VCS on the PSP, not the PS2 so I can't comment really on that. I like all the GTA games anyway so if anything I'd just advise you to try both.
If it's any help, you can probably get a better idea on each game (I don't want to spoil the plots of each or anything) from reviews.