This has been covered so many times, but I have owned both if I had to choose one it would be the 996 tt.

Love the looks of the 993 but the seriously outdated interior does not work for me.

 

I had a 993 tt, 996 tt and a 996 GT2. Sold em a year ago, but if I had to choose which one I would buy back it would be the 993 hands down.

Although the 996s were a better overall performers, the 993 had something that no 996 will ever replicate. Personality.

Also the quality of the 993s were by far better than the 996s.

 

I had a 993TT for 5 years and recently bought a GT2. My reasoning was that everytime I drove the car, it felt old... 1980's old. My TT had 13k miles when I sold it. It was tight and great looking from the outside but dated on the inside no matter what I did with upgrades.
More importantly, the GT2 is in another league with regard to every aspect of performance. It also doesn't ride much firmer, the a/c is much better and offers more leg room. Everone always talks about the switch gear in the 996 being cheap. Has anyone looked at the 1980's window switches, a/c controls & turn signal stalks in a 993?
I can say that reliability was nearly perfect with both 993's I've owned & only time will tell with the GT2.