Depends on where you want to go, along with the reasons given above. If you want to get off on backroads to spectacular sights and leave the mobs of people in the parks behind, then you want 4WD. As for towing a 5ver out on some of these roads, I'd want to drive that road first in the truck to get a look at the road condition first.
For example I'd really like to camp at a spot called Dance Hall Rock
which is a spectacular sandstone rock a couple of hundred feet tall. That hollow was the place pioneer's camped when the men cut a path thru to the river. They held dances there and it's 45 miles down a isolated dirt road. If you climb to the top of the rock you'll see things like this:

And there will be very few others around when you are there. We were all alone and saw one other truck when we got to the end of the road 30 miles further along (Hole in the Rock)
If that's the sort of thing you want to see then get a 4WD