Here's my take on a few of the things you mentioned.
The Marmot is an extra $100. Is it worth it you?
Looking at the EN ratings, Marmot's 30º rating is pretty good. Perhaps, before the EN standard, they were using less fill in this bag so it was more of a 40º bag. On paper, it looks good.
I'll go out on a limb and say the Marmot will compress more. Mostly because down compresses better than synthetics. But with 600 fill down, I don't think these bags will be vastly different in their compressibility. What you will notice, though, is the Wasatch will wear down from compression much faster than the Marmot synthetics are not as resilient as down, so it will wear faster.
I've never liked liners. Instead, wear the layers that you already have packed. Especially if you have good puffy garments. Liners, in my experience, are more hype and placebo than actual warmth.
If you're looking to save the $100, and especially if you don't camp too often in warm weather, the North Face isn't a bad bag. Overall, it's a solid bag, and a great value at its price. The Marmot is likely a better bag, but not $100 better.
Phil and Kevin made good points about a 40º bag, but ultimately, your situations may be different from theirs. Where you go affects what you should take. 40º bags have their place, but, as they say, you may be better off with a 30º bag after all.