I owned both the MB Spiral Down #3 and the WM Summerlite at the same time while trying to find a 3 season Sierra area sleeping bag.
I concur with Will's finding regarding the MB bag, and felt the Summerlite was warmer. I was able to use both bags on consecutive nights in Little Yosemite Valley in 40 degree temps. The 1st night, I used the summerlite under a tarp, inside a Tigoat bivy, on a GG 1/8" thinlite under a short NeoAir. I woke up and had to fully unzip and quilt the summerlite to regulate my temperature.
2nd night I used the MB in a Tarptent Rainbow, on the same pad system, minus the Bivy(I was testing new stuff) I was just comfortable. Not too warm, not too cold.
I also used the MB on Mt Shasta, on snow in June, in temps that touched freezing. I slept both exposed in just a bivy, on the thinlite plus NeoAir, and also in a the TT Rainbow, in the bivy + pads. My feet got cold, and I added my MB UL Down Inner parka, but I believe I was cold mainly due to my inadequate pad combination, only R-value 3 on frozen ground.
While the Summerlite was definitely the warmer of the two, I would have chosen the MB due to the spacious interior. I'm a 3/4 stomach/side sleeper that moves around a bit in my sleep. The Summerlite had just enough room, but the MB was perfect.
I also prefered the feel of the MB fabric, which has a bit of texture, vs the slick microlight WM fabric.
I got a Nunatak Arc Specialist on gear swap, which superceded both those bags.