This is something that will start arguments...but here is my OPINION. (hopefully, anyone who disagrees will note the word in all caps! lol)
The 5.7 litre diesel...good point, great on fuel. Bad point, they do not last. If you find a truck with one of these in it, I would be stunned. Most of them have blown up and been replaced along time ago.
the 6.2 litre diesel. Pretty dependable with routine maintenance. (oil changes, filters, etc.) I know a guy with 265K miles on his, still going strong. I have 2, one of them has 135K on it an runs great. They get pretty good mileage too. Their only down side is that they are a low horsepower engine...so if you are looking to run tractor pulls or pull heavy trailers or something, pick a different engine. People throw turbo kits on these, but it is really a waste of time. You get a bit of a horsepower increase for higher fuel consumption and shorter engine life. Again, this is my opinion. (although, I have been curious if a propane kit might help....)
6.5 liter. Larger version of the 6.2, but beefed up to handle a turbo and higher horsepower. If you are looking for more power, this is the cheap option. (Duramax being the expensive one.)
Truly it depends on what you want to do with your truck with a diesel. We have used our 83 (6.2L) to pull a 4 horse trailer with mixed results. loaded with hay, on mostly flat roads, it did fine. With 2 draft horses inside, it did ok, until we hit a hill, then it dropped from 60 to 20 MPH really quick. I have a friend who has a 6.5 turbo, and his truck goes pretty dang good., even pulling his camp trailer up and down logging roads.
If you are looking for more power, here are 2 options. 1: find a duramax. 2, try this:
http://www.youtube.com/watch?v=K3yohoCqD2E . That is a 6V-53T Detroit diesel out of a medium duty truck. Totally cool!
Cheers!
Offio