33's maybe 34's- 35's require 3" BL
I've heard it both ways, but as you will definitely hear it depends on the truck and tires.
33's will be hell on your trans unless you regear.
People have 35s with 5" of lift. You're going for 6". Put 35s on it, trim what you need to, get at least 4.56s and have fun.
So are you doing this or an SAS?
A solid axle swap is worth the money though.
Axle
Pros are, it's stronger, it will actually flex, there are many ways to set it up and you can have as much lift as you want.
Cons are, it's expensive...