I owned a Mont-Bell Spiral Down Hugger #3, and got a Nitro 30 for my son (On sale at Moontrail for $189, they are out, but they will carry it again but at $219). I tried the Nitro and found it much warmer than my Spiral DH #3, but it was more cramped. It does have some stretch technology, but it does not stretch as much as the Mont-Bell.
I am about 5' 7" and weigh 220 lbs, so I am pretty wide at the shoulders. If you are a back sleeper, then the Nitro 30 may work for you. but if you are a side or stomach sleeper, forget it. I was barely able to wear a Ibex LS shirt and a Mont-bell Thermawrap jacket in the Nitro. I do wish I had bought 2 at that price, but I sent the Spiral Down Hugger #3 back and ordered a MB UL Super Stretch #2, which should have even more room than the Spiral and be a lot warmer (14 oz Down versus 10 oz), but weighs 9 oz more than the #3 and 3 oz more than the nitro 30.