Here's my feelings on them, and I won't deny being bias. Also consider that I'm from Alaska and these rigs regularly see below zero temperatures and even down to -20s:
I have a '96 k1500 and a '96 k2500, my family brought them home from the dealership the same night brand new. The 1/2 ton has the l31 (350 vortec) and has been my DD, it's got about 182k on it at the moment. The 3/4 ton has the second release of the 6.5l for that year, after GM fixed the cracking heads problems. It was my dad's DD up until 2007, and is now my DD when I am in AK for the summer and December. It has about 145k on it right now. It also carried around an 800lb plow on it since the day it came home from about October-April.
My 350 has gotten about 15mpg its whole life as an average between city/highway and my driving style. It's more around 13 now (last 3 years) that I live at 4500' though and drive in the mountains regularly. I replaced an EGR valve last night due to an intermittent code that started a few months ago. That's the ONLY work that has been done to that engine it's entire life. Obviously the oil has been changed, spark plugs, etc.. It runs smooth, quiet, has no ticks, has never burned any oil, and the oil filters are clean every time I cut them open.
The Detroit in the 3/4 ton has been abused a bit more. It's been worked hard as a plow truck and towed a 7000lb trailer through the mountains frequently. The cold that it deals with in Anchorage is obviously harder on it that it was on the 350 because it's an old diesel. Both trucks are plugged in every night it gets below 20, but diesels just don't like cold as much, and its obviously going to be more stress on the motor to cold start like that. The plow truck has gotten about 14mpg it's entire life whether it was driving downhill with a tailwind, or pulling that heavy trailer with the plow on the front. It also has always had trouble getting up to 65mph on the highway (it gets there, just not in a hurry), it's got NO horsepower, but about the same amount of torque as the l31. Although the motor still runs good, it has become harder and harder to start in the cold every year. My dad finally replaced it because he was having trouble getting to work on cold mornings. It started to become a common occurrence to have to run a catalytic heater under the block in conjunction with the electric block heater to get it started on the really cold mornings. At this point it is also on its third starter, and last summer the ridiculous vacuum pump died so I replaced the turbo controller with a mechanical one (works great now BTW, and only cost about $100, no biggie to do that). It also is starting to need the rear main done.
A very close friend that used to be my dad's business partner also brought home an l31 in a 3/4 ton with a NV4500 that same night at the dealership. He doesn't take care of his stuff anywhere close to as well as my dad and I. He changes the oil every few months with cheap stuff, but my dad and I run Royal Purple and Ammzoil and change it at 2500 mile intervals. We run Seafoam through the fuel several times a year, etc. Our friend's truck has about 150k on it right now, needs a quart every few months, but still starts every try and runs smooth. It tows a trailer a bit heavier than ours, and in my opinion has an easier time doing it on the highway. Hard to say, though, his trailer is WAY more aerodynamic than ours, and I'm sure the NV4500 vs the 4l80e makes a difference too. One thing I will say, is that our diesel might drop to 12mpg when it's towing, whereas our friend's l31 drops to around 8mpg. On the other hand, with the stick, it gets about 17mpg on average vs the 15mpg my 4l60e gets even though the 3/4 ton is obviously heavier than my 1/2 ton. This truck is now the crew truck for the roofers, so I drive it back and forth to the jobs pretty much every day I'm in Alaska working. I put a starter solenoid in it last summer and I think it melted an intake manifold around 20k, but to my knowledge it hasn't had any other significant work done with it.
I'm not going to say that the l31 is a better motor than the 6.5l. I think our diesel has been an excellent and very low maintenance truck considering what it's been through and it's age. For a DD however, given the option, I would prefer the l31. I'm perfectly happy with the 6.5l that is my DD when I am home, but if I had purchased the truck, I would have taken the l31 for less money and less hassle. I am however, strongly considering a 6.5l for my wheeler when the 350 in it dies, if that gives you an idea of my feelings on it. It's a great motor, it just needs more care put into it to be as reliable in the cold as the l31. If I had the time to go through a 6.5l and build it simple and the way I'd like it, I am confident I could make it just as, if not more reliable than my l31. I just don't have time to fiddle with my DDs, and every l31 I've dealt with over the years has been nothing short of awesome for reliability. They both run like tops compared to a lot of what's on the market, it's just really, really hard to beat an l31 that was taken care of and left in stock form.
That's my spiel about the motors, I'm not really trying to steer you to one truck or the other. The seller's care put into the truck over the years obviously means more than just about anything, and I'd probably be more inclined to take a 3/4 ton over a 1/2 ton. Is the 2500 a HD GVW (does it have a full-float rear, probably 4.10s then)? The 4.10s in my diesel probably don't help the mileage, but I think it needs them when it's towing the trailer. I also don't care for the gearing in the 4l80 with the diesel, I think they are poorly matched. On the other hand, the 4l80 is a damn tough auto, and the plowing ours has gone through is pretty rough on it. The 4l60 is good in stock form, and can be easily made a lot stronger, but it's still like comparing a th350 to a th400.
Assuming that's a fair price for your market, they both sound like good buys. I doubt either rig would need major maintenance soon unless they weren't cared for well. One thing I like about buying from the owner instead of a dealer is you have a chance to meet the guy and draw some conclusions on how well he maintained the thing. That said, I might take the diesel in your position. If it's got the full-float rear, you'll probably never have to touch it short of changing the oil. While the 10 bolt in the back of my 1/2 ton has never even had bearings and seals at 182k, I'm sure it would require more maintenance in a suburban than a full-floater. I think with the weight of the suburban, and I'm assuming it's an auto, I think the diesel would probably get better mileage. Probably not a big difference between the two though...is diesel way more expensive per gallon in your area like it has been in AK for a few years? That could also be a deal-breaker for me I suppose. I was paying $5.15 a gallon in Anchorage a couple years back.
