Basically it would very much depend on your use. If u drive mainly in town and in traffic condition you would be better off with a 318. But if u do a lot of highway runs, then a 325 would be better.
Also if it is ur first Bimmer, I would advise u to get a 318 so u can get used to the maintenance.
U can find a lot of 318 around so try to get the later model ie 1998 or 1999 which has leather seat , two airbag and digital a/c.
Good Luck.