If this pack is comfortable for you when loaded with your gear (by far the most important factor when selecting a pack!), I'd try a few trips first. Since it's from REI, you can take it back after a few trips or even a few years if it doesn't work out when you're out on the trail. Don't worry, that unlimited return feature is included in their prices!

70L is an awfully big pack! Unless you're going to be sherpa-ing with children, you'll probably never need that capacity. Unless the pack has a terrific compression system to hold smaller loads properly, too big a pack will end up being off-balance. My son has that big a pack, but he carries a big bulky wet suit (he's a surfer and most of his trips are to the Olympic Peninsula coast) plus most of the food for 5 or 6 (in the required bear canister) plus part of the younger kids' gear.

The 55L pack, on the other hand, might be a good idea, particularly if the 45 L is a bit too snug just with overnight gear. You do need more pack capacity when going out in the winter!