I've tried both Air M and a velcro short binder, 3rd gen mesh one to be exact. The short Air M is comfortable because it has elastic around the ribcage- which makes it also not ideal for larger chests. The long Air M has stomach compression, but rides up very little and compresses much more than the short version. The velcro was my go-to binder for a long time, as I had it first and it bound the firmest (probably just as firm as the long binder, but I prefer shorter binders). However, I grew out of it within 6 months on T because I gained a lot of upper body muscle- causing the velcro too much tension.
My favorite binder once I had to bind much less (combination of small chest and chest development) would be their sports pullover binder. They are very comfortable, bind just enough, and are easy to be active in.
As far as sizing, each binder has a different size index. Do not buy any of their binders using guidelines based on your normal attire. Always measure- if you size wrong, they'll replace it, but you pay the international shipping. For instance, at the same point I could wear a size M of the short Air M, but needed a L for the long Air M, and a XXL for the sports pullover. If you have trouble, their customer service is actually really great. They'll ask for your weight, measurements, and height and recommend a size for you.