The message above by JBrian is right on the money...
The I4 is cheaper to buy & maintain, will get 2-4 better MPG. Cheaper because its easier on tires and drivetrain parts, doesnt have a timing belt, and oil intervals are longer. The I4 has been shown to have VERY, VERY low wear rates via UOA's, my two I4's included.
The V6 however, is the better choice if you like on demand strong power, can afford the up front costs, and slightly higher maintence costs. MPG differences are very minor IMO, mostly depends on your right foot.
If I could do it over again, I'd buy the EX-L V6. But for what I spent on my 04 EX I4, I'm VERY happy. There's always a trade off!
