Diesels usually will go for more miles without requiring major engine work. However, that said, the condition of the vehicle that you are considering is far more important. If a gasoline engine was well taken care of and has documented oil changes on a regular basis, and was not driven like a racecar, there is every likelyhood that the engine can go 250K miles without major engine work. On the flip side, there are diesels that need major engine work at 150K miles because they were not very well taken care of.

My own SD needs a head gasket at 180K. 20 year old head gasket finally gave up the ghost. This could be considered major engine work if you had to have a mechanic do it.

I would drive both the diesel and the gasoline versions first. If you find that you prefer one over the other from a driveability standpoint then select the best example that you can afford. The longevity of the diesel should not be your primary consideration.