For pure toy that will be mostly street driven, I like the RT/10. For track I would get a GTS, though other than aerodynamics, there isn't any difference between the RT/10 and the GTS even for that. The GTS / ACR has 10 more hp that can easily be gained on the standard car from a intake tube and filter modification and the ACR has a stiffer suspension that would probably be better for the track and rougher for the street. Also ACR's were lighter due to deleteing things like fog lights and radio, but most buyers got an optional package that added these things back in anyway.
There is a good info area on this site that lists the differences by model year. The things that I would think about in model year differences are:
GEN II (97 and newer RT/10 and all GTS) have significant changes to the car. You can look up the details, but GEN II cars are the way I would go. GEN I are still great cars, but I like the advances made with the GEN II.
'98 and newer RT/10's (all GTS's) have duct in the hood and vents over the front wheels in the hood - I like this look
'99 and newer have 18" wheels
'99 and older have forged pistons and slightly more aggressive cam
'01 and newer have ABS
Probably the biggest decision other than RT/10 vs GTS is what color! Normally this is the last thing on my list when buying a car, but to me the differences are so slight in the GEN II cars that color can be a discriminator. This is especially true since you can probably find any year car that you like in any color made that year, with very low miles.
I went with the RT/10 because I love the ability to drive with an open top, but I think I would have been very happy with GTS as well.
Good luck with your hunt and decision.