rear bag differences - RE7's or RE6's??

[MMM...BRAINS...]

First off, this site is great! i've learned a ton just searching and reading.
I'm new to the "truck world". I recently pick up a 1960 Chevy Apache Longbed. My focus right now is suspension (air) and steering/handling.
i'm not planning on laying it on ground,....but i want to be a close as i can when aired out. Mainly i don't want to HIT the ground if i have bag,line, or tire failure. but anyways,...
this will be my first bag set-up on a vehicle so i'm trying to soak up all the info i can. so, on to the question...

(i'm planning on running Slams)
i know some of this may be personal preference, but what are the pro/cons of either RE7's to RE6'S in the back?
i've read alot of people on here running RE7's on all 4 courners, and some running 7's up front and 6's out back...
let me know.

thanks in advance,
Chris