2500-
All years are great. 6.0 or 8.1. As some said, only real difference are the creature comforts through the years, the running gear is much the less the same. Only small stuff like materials on the gaskets of the front diff, etc. For 2500 GMT 800 the most important thing is the condition of the truck, even more so than mileage.
Short- 2500 are much hardier and stronger than the 1500. If you don't want to touch anything, other than slightly larger tires AND you tow>5000 lbs frequently, than go 2500 with 6.0 and don't look back.
1500-
In my opinion, usually the better option.
Short- Price will allow a ton of mods to get you close to 2500 reliability(and by that I mean breaking things off road, not maintenance failures). You don't need 6.0(much the less 8.1 power) if you aren't towing and the mpg boost for normal long distance travel is awesome. Coil rear end feels MUCH better off road (and on). Does fine towing ~6k lbs as well, mpg is about the same as a 2500 for the towing and significantly better everywhere else, just not as "comfy power" as the 2500.
You can bolt in a 2500 14 bolt, if you are worried about the rear end, and with a lift a 9.25 front end is easy as well.
In terms of years, the 04 or greater is the best option. More power in 2004 and most importantly, hydroboost for the brakes
For the transmissions:
4l60E-
Rebuilds are cheap Quality rebuilds locally in CA are 1000-1500 and will last 100k miles with a tranny cooler.
Agreed 210 is too hot for a 4l60E unless going up a grade in hot weather. For the 4L60E, it's all about the temperature for longevity.
I installed the extra large Tru-Cool cooler in my truck, with the radiator circulation intact. My transmission temperature warms up to 140 at idle and up to 168-179 when driving. Under steep, steep long grades in the heat, it goes up to 190. I haven't seen it higher than that, on 35's with camping equipment, and towing 6k, even in 110 heat in slow traffic on I-5.
4l80E-
Absolutely bulletproof, don't even worry about the temps if less than 220.
There are now kits to put in 4l80E's in 1500's pretty much bolt in and plug in. Probably go with that next time around because I am on all 2500 running gear now.
In the end they are both great trucks. Some of the decision is probably budget, and then the area you want the truck to focus on. With the only except of if you plan on towing >6000 lbs regularly (then you have to go 2500), both will do the job great and reliably with minumum $. In then end for <10k (and much less than pretty much any truck out there you can setup the GMT800 1500 or 2500 to be great off road and on road mileage eaters, around the town haulers and towers. Can't really go wrong.
This comes from owning between myself and my parents over the last few decades havinga GMT-400(from new to 200k miles and sold, still running by a high school kid down the street), GMT800 1500w/5.3(Still have from new at 240k miles and just beginning its off road life at 180k- see my sig.), 1500w/6.0(owned from 100k-200k) and a 2500/6.0(100k-200k)
The GMT800 have been the single most reliable vehicle we have ever owned. It has only ever stranded us once, when a wheel bearing failed at 224k miles after I decided to do a trip as soon as installing a lift kit and not waiting for the new bearings to be be delivered. Only other problems are the fuel pumps start to whine ~10k-20k miles before going out, as well as the transmissions, but they start slipping and you know when before they go.
