The 4-speed automatic in the Suburban really hasn't bothered me in the least. It's got a nice torquey V8 that makes power all across the rev range, so I don't really see the need for the two extra gears.
I've never driven the six-speed, so I can't compare first-hand, but I have heard complaints from six-speed owners that the constant up- and down-shifting of that transmission gets pretty annoying.
When cruising with the four-speed, the thing pretty much just stays in fourth no matter what (until you get to the really steep hills, of course). With the six-speed, even a good headwind on level ground will apparently cause the transmission to hunt back and forth between fifth and sixth.
Anyway, the new Avalanche is certainly a huge improvement over the old one. Big

to GM on this one.