I've been tinkering with the ARC 536 for a couple of days now and am quite pleased with it. Yes, just like any other new software there have been bugs to come up (which is the whole reason for beta testing), but Gommert has dealt with them quickly.
There are things Uniden's Sentinel does, namely update scanner firmware and master database that ARC probably won't. ARC allows you to do virtual control (Pro version) with history logging, along with a little easier editing of Favorites Lists.
Also and this is a "biggie" (at least for me). When you import RR data with ARC, you have a choice of importing the Alpha tag for a channel as the channel name, or the channel description for a channel name.
Yet another good feature of ARC is it will allow you to import files for your other scanners that you have used other ARC products to create. So that means systems I created for my 396XT/996XT using ARC XT can be imported to ARC 536. (However right this moment the GPS data doesn't carry over, but I'm sure Gommert will deal with that).
So for the future, I see me doing a few things with Sentinel and a lot of things with ARC.