I doubt you'd go wrong with either one. Before I was seduced by Western Mountaineering bags, I used a Marmot Helium and Hydrogen, and was quite pleased with both. However, after I bought my first WM bag, I never looked back. (I have a Megalite, Mitylite, and Alpinlite.) There is no difference in quality; both are well-made. However, I've found that Marmot bags tend to be pretty much dead on in the temperature ratings, while the WM bags tend to be about 5 degrees conservative. And, for some reason, they just "feel" better.

Beyond that, as others said, choose the bag that fits you the best. I was lucky; both the Marmot and Western Mountaineering fit me well. I looked at the Ultralight, but chose the Alpinlite because it was a bit larger in the chest and shoulders.

When deciding which bag fits, don't forget to consider whether you'll ever want to wear additional clothing inside the bag. A primary motivating factor in wanting the slightly larger bag was the ability to wear a down jacket and down pants inside the bag, to extend the range if I needed to; you may not be following a similar strategy.

But, in the end, get the one that fits you best. You'll like either one.