Both offer excellent scanners. The bottom line is going to be how many and what kind of bells and whistles you want. GRE is in the lead with that right now with their PSR500 (600 for mobile unit).
I personally feel the 500's front end overloads too easily and it does not let me easily scan the way I want to.
Right now GRE is keeping updates coming out about once every 6 to 8 weeks. I'm not sure if this is an indication of really good customer support or fixing problems with their scanners. I really believe it is a mixture of both. Uniden is much much slower in issuing updates, even for fairly serious system glitches.
Uniden does have a direct rep (UPman) on the board who is very knowledgeable and helpful. But that doesn't help make a decision on what to buy.
There are numerous other threads on this topic. Not only in this forum, but in the GRE and Uniden one's as well. But I can assure you this will draw numerous comments too.
My choice for a front line scanner is the Unidens 396, it better fits my personal scanning needs. I have both the 396 and the PSR 500 by GRE. I'll use the 500 for broad searches or just scanning specific systems. I use the 396 when I need to listen to specific talk groups or sets of talk groups. The manner the 396 allows grouping is (at least to me) far superior to GRE's 'scanlist'. GRE's OOE programming does offer some advantages for some larger complex systems, but I don't have that problem where I live.
Seriously, do some searches and read the other threads, on comparisons. Be prepared for some strongly worded "the ### is the best scanner on the market" (you fill in the numbers). Keep in mind the latest scanners from Uniden and GRE are significantly different from previous scanners. There is a fairly significant learning curve, but in the long run, either will probably meet your needs.
This is like asking: Chevy or Ford, caffeine or decaffeinated, cream or black,..... It will all boil down to what you read and your own personal decision as LEH has stated.
I preferred the Uniden because of a particular frequency I monitor in my area, it is in the 70 MHz range.
GRE does not cover that range where Uniden does, that was a big factor in my decision.