Ah, you're going to get so many opinions.
Is there something your PSR-600 doesn't do that you want it to that is causing you to consider a new radio?
I will tell you that the last 3 scanners I bought from Uniden (396, 436, and 536) all had to be shipped back for defect repair (recalls). In contrast, I've never had to return any GRE or Whister radios for design defects.
I have both the ProVoice and DMR upgrade on my 436. The ProVoice option seems to work fairly well but there's no ProVoice in my local area and the number of ProVoice systems will slowly fade over time. I still can't get DMR to work very well on my 436. In contrast, my Whistlers are very active and hearing DMR. I find DMR programming on the Whistlers to be much easier and the programming software actually works and let's you programmed from the RRDB as well as manual programming.
Also keep in mind that there is a "base" cost for the x36 radios and then an additional $50+ for upgrades to each of ProVoice and DMR (the latter of which Uniden has warned/threatened in the last week will have 2 price increases between now and the end of the year). You can probably expect another dip into your pockets for a future NXDN upgrade. Upgrades for all 3 may cost you an additional $100-200 by the time they are all done.
If you stay with Whistler and purchase a TRX-1 or TRX-2 model, you'll get DMR and NXDN for the base price of the radio (no "per feature" asking for more money - at least none that I no of at this point).
Given the future fade of EDACS and ProVoice, if you haven't had a need for it yet, it's not likely you will in the future.
Assuming you've been happy with your PSR-600 and are considering your options. you'll want to look at the features of the Uniden models and the Whistler models and decide what you really want/need.
Good luck!