search the forum, there are a few threads on this. I started one a couple of months ago. Feature comparison is easy - go donwload the manuals and check. For me the real question ultimately hinged on voice quality and as an added bonus (since it is so hard to do) on speakerphone quality. That resulted in me purchasing Polycom 501 phones. At the time (and probably still) Cisco licensed their speaker phone technology from Polycom, so that tells you something. Also at the time - in the tread that I started, someone had just come back with a side by side demo of the Polycom and Snom speakerphones with a customer - the Polycom won hands down. (Look for the thread to get the details).
All three are probably excellent choices, as well as I’ve heard a lot of positive on the Aastra phones. So it is going to be your priorities and to some extent a price/performance comparison if that is important to you. (When I was looking - the Polycom 501 could be had for about $175).