The design of the 70L is perfectly suited for airplane carry-on or check in, and the features of the 70L are perfect for any adventure once you arrive at your destination. Being able to compress the 70L allows you to use it as carry-on for all major airlines in the US and Canada.
Trips of 5 days or more usually call for packs of 70 liters or more. These are also usually the preferred choice for winter treks lasting more than 1 night. (Larger packs can more comfortably accommodate extra clothing, a warmer sleeping bag and a 4-season tent, which typically includes extra poles.)
No matter how well-designed the backpack, less weight is always better. Use the bathroom scale to check that a pack isn't over 10% to 20% of your body weight (for example, if you weigh 120 pounds you shouldn't carry a backpack that weighs more than 12 to 24 pounds).
70L is definitely too big. I believe the 55L is a 40L + a 15L daypack. This will probably work, but double check the length, width, and height, and make sure none of those dimensions are larger than allowed for a carry on.
If you will potentially need that extra capacity on a future trip, then definitely get the 75L pack. If you're doing it just because it doesn't seem like much extra weight, even if you don't expect to ever need it, I would go for the 65L pack. That would also curb the temptation to overpack.
As a rule of thumb, 25-30 liter backpacks are best for shorter weekend trips, while 30-45 liter backpacks are best for longer trips or long-term travel.