The atom appears to be something really special. I have been following audiosciencereview for a few months now, and those measurements are really something. Proof of good engineering. That is what you are buying. Arguably you don't "need" such good noise/distortion numbers, at or below the threshold of audibility. Amir, the main guy at audioscience, himself uses the Topping Dx3 Pro as his main DAC/amp. This is with all the stuff he has at his disposal. Its numbers aren't quite as good, but he likes that it is a one-piece unit rather than a stack, and has convenience features, and is small and cute. You see what I'm getting at?
The O2 was designed to prove that you can make a competent, class-leading amp with $30-40 worth of parts. And it was. Even now, it holds its own, but is uses older parts. If it was me, I would chose the Atom in a heartbeat. State of the f'Art and all. Just know that you would probably not hear that much difference, if at all, especially at normal listening levels.