The overall breathing differences will be of no significant difference. The Abyss offers a larger second stage with a corresponding increase in weight. The Abyss uses very few O-rings, thereby increasing its overall reliability. However, it is more difficult to fine tune due to there not being an access port in the side. I personally tune them by holding the case sections together and then checking the setting after fastening the two pieces together with the lock ring. Of course, I've also been tuning this style of reg for many years...
The Proton offers a tiny second stage, which some divers prefer. The tiny exhaust port creates more of a problem with bubbles in the diver's vision. If you are swimming forward in a horizontal position, this is not a major problem. Yikes, I miss the larger, swept-back tees! If you like the color scheme of the Ice, then so be it. Mares will soon be shipping the Proton Metal model (some already arrived), which offers a more familiar look. I do like the fact that the Proton can be stripped while underwater.
The VAD (vortex assisted design) does work very effectively in making the Mares design one of the best breathing non-adjustable regulators. While it doesn't have the fine tuning that adjustable regs offer, it does utilize the simple benefits of hands free operation. I've owned and used many adjustable regs over the years. Sure, they can be adjusted to offer lower cracking effort, but I am more than happy with my Mares regs down to 150' on air. Below that helium mixes should be flowing so the effort is less than that of air.
The first stages on both regs are the same. The Proton's merely has excess brass trimmed away for a more modern look. The internals are exactly the same. That particular first stage provides more gas flow than three divers could need at 150' or more.
I can give you specific bench test data if you should need it.
Greg Barlow