I love my WM Caribou bag. Great construction, super roomy and very lightweight. I think the 35 deg rating is about right. I've never been cold in this bag but then again I use a 20 deg bag if the temps are expected to drop below freezing.
The Megalite is definitely going to be a warmer bag just given it's baffle construction and additional down. If you are trying to stretch one bag for all of your backpacking needs, then I'd go with the Megalite as you could use this in a wider range of temps. If you already have a cold weather bag and are looking for a second bag, then save some dollars and go with the Caribou.