I'd say if you want massive amounts of power for cheap you vet a V8
If you want massive amounts of MPG you get an I4
If you just DNGAF, get a V6

Yes I4s can make power, and V8s can get great gas mileage, V6 engines can do both. It's more dependent on what you're sticking it in and what you'll be using it for. I'm sure I can put like 30k into my 3.1 and have it push out like 1,000hp. But why bother when you can do the same thing with a V8 for about 4-5k?
If I want driveshaft snapping torque, I'd get a V8... If I want excellent gas mileage, I'd get an I4... If I just didn't give a crap about either, I'd have a V6 (which I have two of... blah).