They are both great. I use Zeagles, but a lot of my buddies choose Atomic. When I was doing my Intro to Tech class, and we were doing a lot of air share drills, we both commented that we couldn't tell the difference in breathing quality.
The atomic is a piston reg, the Zeagle is a diaphragm. I prefer diaphragm for cold water, but the piston can be used, but you have to have it setup for cold water, which involves having part of the first stage injected full of grease. I'm not a fan of that setup.
Also, if you ever see yourself doing your own service work, Zeagle is easy to get parts for. Atomic can be a pain.
Jim