I have both the picatinny rail mounted Caldwell Brass Catcher, and the velcro Caldwall brass catcher (both referenced above).
The difference is night and day, although I do not think that the rail mounted is worth $45, when I have found them on sale for $24, I bought 3.
Each of my ARs (2x 15-22, 1x 556, 1x300 BO, 1x9mm) has the rail mount on it. It's an easy switch out from one platform to the other. Extra rail mounts are available.
As was mentioned, FTF/FTE are issues; also, some ranges want to see the bolt open, so you have to remove it. It's not an issue with the rail mounted version. It's an issue with the velcro version.
I had to jury rig the velco version to stay up with more than a magazine worth of 22 cases because the back end of the bag is not supported. The support structure with the rail mounted is much better; you can also choose where to 'mount' the rail portion to be in front of, or behind, scopes/sights. Can't do that with the velcro version.