DB,
If you are after a daily driver with little to no towing, any of those engines will probably serve just fine. The pluses about the 350 cid is that there are literally millions of them on the road, which means every junk yard has a hundred of them and parts are cheap. They are easy and cheap to modify as well, you can take a small block chevy to 600 horse without having to take out a second mortgage. There is nothing wrong with the 6.5 either, its just not known for its stump breaking torque and horsepower. They also arent as common, so parts and repairs will be higher, dont expect to do much in the way of pulling with it, there is a very valid reason Chevy went to Isuzu to get the Duramax. But if you just have to have a diesel and there is an inexpensive clean low mileage 6.5L in your area then go for it. My dad just picked up a 95 6.5L 4x4 for $6500.00 about 6 months ago to use for his beater pickup on the farm. So far he's been real happy with it and for any serious pulling he uses his 99 24V.