TFox pretty well covered the issue of the Dual vs Single. I happen to think that the single does release a little smoother.

As for other features, I like the convenience of velcro. I also like that you can in theory wash the fabric and it won't absorb smells like leather. Also leather tends to make a creaking sound while drawing as pressure it applied. This nearly cost me a bear once. It heard that leather skrreeeek as I drew and whirled and ran out of the bait site. It did come back and darn if it didn't do it again. Third time he came in I got it back a little quieter or he had just heard it enough he decided not to run, not sure which, but it was his last mistake.
Also shoot them all. Some have different trigger angles. I for one had a hard time adjusting to the backward lying angle of the TruBall releases, but I got the hang of it after a while and now wouldn't shoot anything else. An adjustable trigger angle is avail on some models. Just be sure it fits into or just forward of the first joint of your index finger (from the tip that is).
Of course nearly all of them have an adjustable trigger for travel. Be sure that the jaws close on release of the trigger, don't get a hunting release that you have to push the trigger forward to close the jaw. That is just unhandy.
Remember above all KISS