Some may say it's overkill, but if you're continuously pulling 3.5 tons, I'd go with a 3/4 ton pickup. BTW you may want to find out what the fuel economy of those vehicles will be when you pull that much. I've personally pulled 35,000 lbs of peanuts. I probably don't know anything about this type of thing.
The engine is never the problem with a towing truck.
New tacomas have more power than old 1 tons did.
The transmission, brakes (<--- a really big deal), rear end, suspension, and even the frame if you get into high tongue weights will all suffer on a half ton truck. They're leagues better than they used to be for occasional towing, but if you want something to last for 150k miles with something hooked to it, you need a truck made for towing.
Don't think I agree w/ you re: the diesel fuel economy. My 6.7 F250 is almost identical to my 6.2 GM. 13-14 city, 17-18 hwy. Haven't seen maintenance costs rise, oil changes are more, but you only do them every 10k.