I have 3 hummers . One a 92 and a 98 (as well as some military trucks) and an H2.
The 6.2 is a far more reliable truck than the turbo's and for the most part the 6.2 is a HMMWV with nicer seats and interior.
The passenger heating and cooling on the 6.2 is not very good
The 6.5 trucks, with and without the turbo are far more complex electrically and this is where most of the problems will occur.
The big advantage with the 6.5 trucks is that they have the 4l80- transmission whereas the 6.2 have the original 3L80 3 speed hmmwv transmission. Any faster than 55-60, you are killing the engine.
The turbo trucks have a lot more torque and feel almost like a sports car compared to the 6.2.
The 6.2 trucks have a lot more room inside due to the smaller engine cover (the dog house).
There are some notable differences between the 92/93 Hummer and the HMMWV:
The Hummer has a very different transfer case that is lighter duty for towing but is capable of higher sustained speeds~ this owing to a very differnt planetary gear.
The older HMMWV's had a drive shaft brake, whereas the newer HMMWV's and the hummer have a special rear caliper set - which always corrodes and gets stuck. I think I would prefer the drive shaft type.
The 3 speed HMMWV transmission has no park position - ths owing to the park holding requirement on the HMMWV - The park pawl on the 3L80 transmission cannot hold the weight - they therefore eliminated the park position. All hummers have the park position
The HMMWV's are 24 volt, the Hummers are 12 Volt. There are pro's and cons for 24v. Cold start with 24V is easier but getting a boost to start with 24V is not so easy unless you have another MV around.
If I had to buy one truck I would go for the hummer. If you want comfort get a turbo hummer as they have better sound insulation, heating and cooling. The best year was 97.5 or 98, after which the trucks came with ABS, which further complicated the whole system.
As far as the H2 goes. I have yet to see a situation where the H2 has not performed as well as the H1. Mine has been through rivers - water half way up the doors (which is near the top of the hood on the H1), been in deep mud, sand, snow and up and down very steep gravel hills, all along side H1's.
There are some weak spots on the H2 but all in all, it is not a bad truck. Also despite what many say, the H2 is not built on a yukon or tahoe chassis, rather it is a special build chassis (wider and heavier) that uses SOME yukon drive components. Even the differentials are H2 specific. The Yukon parts are transmission, brakes and from I can see the hubs - thats it.
The H3 is a different story as it is a regular GM truck but with some upgraded components.
And finally, for offroad comfort the hummer (h1 or h2) is second only to the Stalwart (which has a very smooth ride - even at its top speed - a whopping 43 MPH - if you can get it going that fast off road)