We have a 2001 classic 30' which is the same length of the 31' and find the accomodations fine for two. (steamed windows and all). I myself would prefer the longer trailer.
As for tow vehicle, we tow our trailer with a half ton sub. When we purchased the vehicle in 2001 the tow capacity was 10,000 lbs with a 4:10 rear end. We purchased a 3:73 thinking that was good enough for towing and non-towing. After the first year, I switched out the ring and pinon gears to a 4:10 rear end. The difference is unbeleavable. Whatever you do, 6.0 or 8.1, get the 4:10 rear end. Myself, I think the 8.1 will be overkill. We've been through the rockies twice and of course the white mountains here in NH many times and find the 5.3 to handle our load just fine.
Good luck in your decisions.
-Life is a journey, not a destination.