The Marmot Sawtooth is filled with 650 down compared to the 850 fill of the Helium 15. I think I would spend the extra money for the warmth because I run cold at night. In terms of quality they are both excellent bags and exceed quality. It all depends on what you want in terms of warmth. Thanks
The other answers about the 850 fill bag being warmer are not accurately representing the difference in 650 vs 850 fill. Both bags have a similar comfort rating (22.6f for helium and 20.5f for the sawtooth) and the fill power has more to do with how much fill is required to achieve this rating. Based on the comfort tests, the Sawtooth is actually the warmer bag, but it requires more fill to achieve that warmth and as such is heavier (2lb 8oz vs 2lb 1oz for the Helium) and will not compress as much (10x16 vs 7x14 for the Helium). The decision between the two bags should really come down to how much value you place on a lighter weight bag that packs down smaller.
The Marmot Sawtooth is filled with 650 down compared to the 850 fill of the Helium 15. In my opinion well worth the $100 price difference for the additional warmth.