Well i may be biased but me and my family have had great luck with chevys and especially the new ls series motors. I have a 99 GMC Z71 that i beat the crap out of(i do take care of it) and it has never let me down, it now has 141,000 on it and it rides like new and still tows and offroads with the best of them. My dad has a 2000 Z71 and he has 150,000 on his and again no problems at all other than regular maintenance. They are great trcuks and i'm not sure anymore what truck is more heavy duty the 4x4 or the Z but my Z is a great truck
I would get the Z just because it cooler. But the 4x4 and z71 are pretty much the same truck just a few upgrades on the z like skid plates stiffer shocks.