IMO it depends on how you want to use it and how much you want to spend.
If your worried about MPG.. you'd most likely not be happy with anything that would increase power.
For me... I'd go the V8 route. I'd like to take a 3V 4.6L and autotranny out of a explorer. add a set of mild cams, and then work on a tune to make it streetable for daily driving. It would get 20+ mpg and probably run in the mid 14s. Probably could be done for under $5k too.