Sure, I bought the 996T at Dayton last year. It has been a major disappointment. So this year I wanted a handheld, and I bought the GRE PSR-500. It ran rings around the 996T. So a week later I ordered the PSR-600 and it has now replaced the 996T which my son-in-law borrowed but will be going to my nephew soon.
The 996T audio quality is no where near the 600. The 996T sounds muffled and the 600 sounds very clear. The 600 scans much faster. The 996T is prone to front-end overload and/or adjacent channel interference to a much, much larger degree than the 500 and 600. That means with the 996 the digital transmissions are going to be missed or broken up. I hear much more on the 600 than I ever did on the 996 and they are programmed identically with the same frequencies and services.
The rest of the differences are gee wiz things and cosmetics. The real difference and all anyone should care about is that the 600 is simply a better receiver and you will hear more with it than the 996.
Oh and GRE seems to come up with firmware updates more frequently. Uniden is very slow and even their latest firmware update has been a big disappointment meaning it has not done much to improve anything. In fact, my 996T is running with a beta firmware which is better, but the audio is still bassy and the adjacent channel interference or overload issue is still there as I really don't think they can fix those issue with a firmware update.