Diesels are usually very good at minding the fuel economy because of their wide fuel/air ratio. In my experience, a T444E/7.3L PowerStroke engine is almost bulletproof to the point where you have to actually try to kill one by seriously abusing it. Those motors do last a very long time... but the injectors, transmissions, at the like are likely to wear out before the motor does. Also, from what I understand, the E4OD transmission apparently has some aftermarket parts upgrades that also makes them nearly bulletproof as well.
My own previous diesel pickup was a naturally aspirated 7.3L motor. It got somewhere around 18 MPG until very late in it's life. While it would have lasted a while longer, a rebuild would have made it a very good motor again. Also, what I found is that while diesels can be a bit cold-blooded in the morning, if you have a block heater (and if it's really, really cold, an oil heater and fuel tank heater) they start up in the morning really easily. In fact, my pickup could light off with only 3 of it's glow plugs working, even without the block heater.
Gas motors are relatively cheap and will do the job if you need them to, and they have plenty of get up & go at some expense in fuel economy. We had one gas ambulance (and it was from 1983) that was quite peppy still... but it burned a lot of gas for it's performance level. As such, we didn't use it much because it was so expensive to run.
I'm really a good fan of the diesels. They do last a long time and are good for what we'd need them for. We don't need to sprint... we need to run a marathon. Diesels will do that quite nicely and efficiently. That's also why most OTR trucks are diesels. It may take them a little while to get up to speed, but they can cruise all day quite happily.
Oh, and I almost forgot... I once had a VW Rabbit diesel. If I wanted to go somewhere via the freeway, I figure if I started now, I'd be at 65 MPH sometime around next Tuesday, (it was that gutless) but it would be able to stay there all week...