As fate would have it, I am currently playing with both of those models. Since I only have a few dives on each, and I do not decide whether I like a reg until I have at least 50 dives on it, I have not made a decision. I am leaning toward the AT20, since it seems to breath a little better. I have not had either below 100 feet yet, however. Since I no longer dive cold water, the environmental seal is not a big deal. The Cdx5's seal, being a dry type, is much less a PITA than the old alcohol or grease seals were. I played with the adjustment on the GT3 second stage on the first dive, and didn't touch it after that.
I expect to have at least 50 dives on each by July, so if you haven't made a decision by then, PM me for an update. Right now, I prefer my old faithful Titan LX to either.