I'd recommend the 3/4 ton, since it gives you more options should you want to move to a fifth wheel or truck camper.
You can't really go wrong with any engines, although if you go with a 3/4 ton, you will end up with the 6.2L on the Ford or the 6.4 TD, similar with the Chevy and RAM pickups. I always go with the biggest engine available in a vehicle, the only exception are the Ford F150s and the EcoBoost (especially come model year 2015 when the EB will be the top tier engine and the normally aspirated 5.0 be the low end motor.) Except for the Ford engines, there is no substitute for cubes, and the MPG difference is negligible for the most part.
From there, it is choosing between gas or diesel. This is an opinion thing here. I have my own thoughts, but everyone has different wants and needs.
JMHO, Were I looking for a tow vehicle, since 3/4 tons and 1 tons are virtually the same price, and almost the same on the Fords, I'd just go for a one ton.