I don't know. The only person who could really know would be someone who has used both - and even that would not be conclusive. Every bag is unique, and every person is unique.
I had a very hard time finding comparable facts for the two bags, different manufacturers publish different specifications (like I could not find the foot box circumference for the Marmot (M), and I could only find the LONG specs for the North Face (NF). However, the NF seems to have 47oz of 800 FP down compared to 48oz of 800 FP down in the M (both longs). The 44oz mentioned earlier for the M was the regular. Someone mentioned the difference between European and American FP calculations. The European one uses a heaver compression piston, so their measurement is more conservative (750 FP European ~ 850 FP American). However, I could not determine if NF used one standard and M used the other. I DO know that Marmot has all their bags (except this one) tested in Europe for CN rating.
My bottom line review - based on my experience - is this: every North Face bag I've had (I've had 2-3), has seemed cold at or near it's rated temperature. I've tried a Marmot bag and I have a Mountain Hardwear bag. Both worked well (for me) at or even just below its rated temperature. I would buy a Marmot bag over a North Face bag any day of the week, all things being equal. However, to put it into better real-world perspective, I would buy the North Face Inferno over the Marmot if I could save at least $100. I do not need a -40F, I need a good -20F.
Hope this helps!
LOL - three years later and I have the Marmot -40. Unbelievable bag, though I have not used it yet. Think of this though... you can lie on top of it and the loft does not go down. And I'm not a light man... Expect an update after I try it out in northern NH!