How does this compare for water-resistance and wind-resistance to the Arc'teryx Gamma MX Hoody? Which is the most breathable?

I would add that the Gamma MX is a little more of a stand alone piece. it is warmer than the Ultimate and the hood is compatible with a helmet.That being said the Ultimate is a jacket that you will never have to take off. You can wear it without really wearing it due to he venting system and it is a bit more windproof and lighter.

The windstopper fabric doesn't breathe very well. However, the design of this jacket includes full side zips with double zippers and a front zip with double zippers. There are many ways to vent the jacket to prevent overheating. I haven't found breathability to be an issue yet.

The Gamma MX Hoody uses Powershield instead of Windstopper, so it is only wind-resistant instead of windproof like this jacket is. The tradeoff is that the Powershield is more breathable. The advertising for the Gamma MX claims that it blocks >90% of the wind - I'll let you be the judge. As for water resistance, it's hard to say. Neither jacket is inherently waterproof, so you are relying on the DWR coating. I know for a fact that the Gamma MX is pretty impressive in light rain, but I wouldn't want to rely on it for prolonged showers or a downpour.