If you are going to use it in a vehicle, the 996XT is the better choice. I use a PSR600 at work and a 996XT in my own truck.
Here are the big advantages of XT.
Display is much bigger and easier (for me) to read. I think that GRE made a mistake using the same size display for both the PSR 500 and 600.
The controls, volume, squelch, and VFO knobs, all have positive detents and will hold were you put them. The PSR600 doesn't have that and I find that the volume control especially moves way too easily. Also, the XT has an audio taper volume control while the PSR has a linear taper. On the PSR, the volume goes from whisper to ear drum rupturing with very little movement of the volume control.
The PSR seems to be more susceptible to 800 interference, but that might be because we have two 800 Mhz data radio in our vehicles. Still, the BC15s that we had before didn't seem to have that problem.
There are a lot of nice features on the PSR that I wish the XT had, but there are some nice features on the XT that I wish the PSR had.
I actually like both radios and would like to pick up a PSR600 just to have around the house to play with. I have ARC500 so I can program it myself.