Chad:
I think much depends on (1) where you tend to camp and (2) what bags (if any) you have already. As noted, I like the idea of owning two bags — one three-season and one summer bag — with the two used together as a winter bag of sorts.
If you tend to hike higher up where even summer nights are cool, then using a No. 5 or even a No. 3 as your summer bag makes sense. Conversely, if you camp at lower levels — say hiking or canoeing in the South, etc. — then a No. 7 might be a better choice.
If you notice, the regular prices of 725fp and 800fp differ very little. This make sense to me because the perceptible differences between the two are exceedingly small. So now is actually a terrific time to pick up a couple of 725fp bags.
Daniel:
MontBell bags can be unzipped almost all the way. Picture one like a "quilt with a footbox".