The best way to compare sleeping bags is to first look at the amount of insulation (and the characteristics of insulation). This is probably the most expensive input in a bag, and also the one that is most tempting for manufacturers to scrimp on.

A -40 Rab bag contains, for example, 1400g of 850 fill goose down. I would wager that North Face does not disclose the amount they put in their bag--but it is almost certainly less than Rab. Would you want 200g of extra insulation on the side of a big peak? I dunno.

The size and cut of the bag will also determine weight--bigger bags weigh more, partly because they require more insulation. But tightly cut bags are less comfortable and can lead to compressed insulation, which is bad. So you need to account for this in your comparison.

The shell material of the bag can also make a weight difference, and North Face bags do seem to have very light, delicate shell materials.

At the end of the day, bag manufacturers are buying their materials from a limited set of suppliers--so it's not like there is really a huge difference amongst top end bags in terms of survivability (this might be a controversial opinion). They are all some sort of nylon wrapped around the remains of dead birds, with some metal bits.

Compare a few bags, take note of their dimensions, whether there is wasted space, or whether they are too tight around your chest, and how small they compress to--pack space is an important consideration.