My dad has owned a 355 GTS and currently owns a 348GT Competizione. ALthough the cars are vastly different, i'll give you a quick synopsis of what both of us have inferred. (mind you, i never drove the 355, so this is mainly his opinion)
Performance- Not much of a difference between our 348GT and the 355 (400lb weight difference and maybe only 15hp disparity, with that going to the 355 :lol: ). But from what i've heard, the 355 is quite a bit quicker than the 348, mainly due to the better power, but also because of the EXTRA GEAR!
Handling and feel- 348 is more raw-- this opinion is shared with every other owner who have either driven or owned both models.
Reliability- Early 348 models should be avoided...many had electronic and transmission problems (from what i've heard). Our '93 348 GT has had no problems and is driven hard with 36,000 kilometers on it. It can sit for 2 weeks and then start up just fine, which i think is pretty good for an early 90's ferrari. 355 basically has no issues all the way around the board.
Overall, there should be no question. The 355 is hands down a better car than the 348...but at a price. A good 348 can be had for $50k. I think the GTS is the model to have in either form.
Good luck with the purchase.