2.7L is an okay motor. Compared to the 3.4L, it pales in about every single category. I don't know of a single person that went from a 2.7 to 3.4 and regretted it. The MPG is slightly better with the 2.7, but at what cost? Any load, and it struggles (my friend has a 2.7L 5sp Tacoma, so I know this). Gotta wind it up much more to get up to speed. To me, this isn't worth the 2-3mpg's (if that). I get 22MPG on the highway, so that is good enought for me.
As far as the longetivity, I believe the 3.4L will be right up there with the 2.7L. Just take care of it, and it will last.