I got the Ultralite because it has a draft collar, which IMHO is worth at least another 5*F or more worth of insulation.

Before ordering any sleeping bag, measure your girth (measure shoulder girth over your arms and while wearing all your insulating clothing) and compare your measurements to the specs for the sleeping bag. The Ultralite is narrower than most bags. The Alpinlite is WM's wider version of the Ultralite, more comparable in girth to the Marmot Helium.

I was a bit hesitant about the Ultralite, since it's a "unisex" model and I have quite ample (!) hips. Before ordering, I emailed backcountrygear.com with my hip measurements and asked if it would work. They found an employee with similar measurements and she crawled inside the sleeping bag to check for me! And my Ultralite has worked just fine!

If price is a concern, you can probably find the Marmot Helium on sale somewhere.