Practical tow capacities not the extremes or limits?

Either will be fine for your set up. It's ok to slow down on a steep pass for 5 minutes. I like 4x4 because when you. Need it you need it.
The 8.1 has an Allison tranny unless it's in the 3\4 avalanche and maybe the suburban.

I tow 10 k with my 454 4 speed tranny. I do slow on a pass. No big deal. The rest of the time you will like either set up.