If you can get a 70 for about the same price as a 60, get the 70 for reasons Dapepper listed. However, if its gonna be a little more for the 70 then you want to spend and you can get a 60 for a good price, you wont really miss much, not for what sounds like pretty tame off roading. You can just get a couple inches of axle or body lift and put 35-37s on there and be set for pretty much anything short of rock trails. Not that i'd ever take a pickup for more than you'd find on trails in national parks for example.
I agree, i'd just about kill for a 6.4. Personally, if I had to choose between the 5.9 and 6.7, i'd go with the 5.9. You can make plenty of power with a decent amount of upgrades and tuning. But, I value rock solid unwavering reliability over moderate or even double the power increase.
Just my own little bit, I always hear people saying the 5.9CTDs were way more durable than the 5.9 gas, and the 5.9 gas and 5.2 gas are functionally identical, and my old 5.2 was an absolute beast on its deathbed. It got me the last 30 miles home pulling an enclosed trailer 70mph down the highway with absolutely no oil. I can't imagine how tough the 5.9CTD is if it beats that.