Hopefully our fine sponsor ScannerMaster will forgive me for this (what a swell bunch of folks) but this is lifted straight from their PSR500 page:
The GRE is better if these are most important to you:
* Better digital decoding, especially on simulcast systems.
* You prefer a radio with better audio.
* If you prefer separate volume and squelch controls.
* If you don’t want or need the extra features of the BCD996XT
* If you are not bothered by excessive strong-signal interference.
* If you need higher sensitivity.
* You like to have Virtual Scanners available to you.
You will want the BCD996XT if these are more important:
* You prefer a simpler but more detailed display.
* If you want the amazing feature-set of the BCD996XT
* You prefer the Banks/Systems/Groups method of programming.
* You need or want the GPS features.
* You will use the radio in a high interference area.
* You want to have a lot of channels available all the time.
* When fire tone-out capability is important.
* You monitor a large networked system with multiple sites.
Ref:
http://www.scannermaster.com/GRE_PSR_600_Police_Scanner_p/30-501186.htm