If you want to follow P25 Phase 1, the PSR500/600/800 and BCD396XT and BCD996XT and HP-1 will work. However, if you to want discover new systems, the PSR500/600 work great in Tune mode. Likewise the HP-1 with Extreme firmware allows dialing a frequency to see what's there.
I believe Pro96Com supports all but the HP-1 so that's an extra software option.
The HP-1 (with Extreme firmware) is both computer programmable and computer controllable meaning you could use the radio for control channel monitoring or voice following. The downside of the HP-1 is a lack of choices in decoding software - which may change over time.
If you want to monitor P25 X2, the PSR800 is the only option. The '800 is computer programmable but not computer controlled. You must navigate the menus to seek a specific frequency. This model is unique in providing both control channel decoding over the USB cable and optionally provide discriminator audio over the headphone jack. Searching for a signal is a bit clumsy.
If you want to monitor P25 Phase 2, you'll have to wait. Any of the above Phase 1 capable models can be used to decode the control channel but not the voice channels.
The main advantage of discriminator audio is the ability to decode signals the radio's own firmware doesn't understand.