From my experience living in a big city with lots of RF....
The 996XT is really the ONLY choice if you plan to put up a mast / tower for your antenna, and the PSR-600 is the better choice if you plan on only using an antenna attached to the scanner.
The 996XT has the selectivity to handle the strong RF, while having pretty good sensitivity.
The GRE scanner is more sensitive for sure, and a good deal better on simulcast digital systems, but is nearly worthless with my outdoor antenna which is 40' up, at least here in the big city. Intermod, intermod, intermod. The Uniden has nearly zero issues with intermod. And no, attenuation is not the answer for the most part, at least in my area. 20db is just too much attenuation for anything UHF and up, and doesn't always work on VHF much of the time either.
I would say that if digital simulcast systems are your main priority, you might be better off with the GRE unit and the stock antenna. If you scan aircraft frequencies, and are interested in distant analog VHF/UHF, get the Uniden. I own several of each, and love them both for what they are.
If a person was to plan on having one base and one portable, I would suggest the 996XT for a base with a good outdoor antenna as high as possible, and a PSR-500 / PRO-106 for a portable.