I'm a GM guy through and through. My tow rigs (for my X-45) have gone from an '02 Escalade (6.0) to an '02 Suburban (5.3L), now to an '05 Escalade ESV (6.0L). When my rig isn't available (wife is toting kids around), I have my buddy pull it with his Tahoe. The trucks have never let us down and the longer wheelbase in the 'burb and Escalade is quite helpful when pulling long distances in variable conditions. I will say that I am a bit slower over mountain passes with the 5.3L, for obvious reasons, and when my Suburban ventured above 175,000 miles this spring I knew it was time to move on.
My vote is GM, but then again I am biased.
'06 8.1L X45