I am using 22ct for my Lady & Unicorn (and have used it for lots of other things, as well). I use the Charles Craft brand that I can get locally at Hobby Lobby, JoAnn's, or Michaels - any of the big box stores that carry stitching (not sure where in the world you are - but you should be able to get it from numerous online shops if you are outside the US). I can't compare other brands - I don't use enough aida of any count to have a preference (other than I rarely use aida for anything anymore, except full-coverage pieces like SQ).
I chose it primarily to keep the finished piece size down - framing is EXPENSIVE! As a side effect, it does make the detail very sharp. In deep confetti areas (which is pretty much ALL of the Lady & Unicorn series), it can be tight to get the 2 strands through, but I have always used 2 strands on 22ct, even for non-SQ designs - I just tend to bend/break more needles on L&U
I am starting another SQ in a week or two, and this one, I've chosen a 20ct aida - have never used it before, no idea who the maker is; WIchelt, probably. I got it at 1-2-3 Stitch:
http://www.123stitch.com/ I don't expect it to be much different from the 22ct. To the naked eye, I can see zero difference - we'll see if that's true when I actually put my first stitches in
By way of comparison, I have a set of Santas that I did - it is the exact same pattern, in 3 different colors. I did each one on a different count of fabric. The Red Santa is on 14ct, the Yellow Santa is on 18ct, the Blue Santa is on 22 - maybe this will help you compare:
https://www.flickr.com/photos/rifestitch/8330846391/in/set-72157632398646146
Hope this helps! What are you working on or planning to work on?