Get the V8. I test drove several V6s and the fist time I drove the V8, I was hooked. It is faster, much quieter and smoother, and the torque helps when going up these mountains and trails in Colorado. The MPG difference is negligible. The last several tanks have gotten me as low as 17.5 when mostly driving near/in the city and 19-21 on the highway at 65-80. As I go up, the MPG drops. I think it hits about 2100 RPM at 70 MPH. That is nice and low, and keeps the engine silent.
It's also probably Toyota's most reliable engine, and is extremely easy on engine oil. I have driven the new 4Runner too, and I still prefer the V8 4th Gen by a wide margin. I was tempted to sell it since I recently got my new Rubicon but I realized how dumb that would be, so the Subaru gets sold instead, even though it is the most economical and also very reliable. I would really regret selling this one, and finding such a pristine and babied one again would not be easy. If I had a V6 4Runner, I would have sold it instead of the Subaru. That V8 is so nice, and the ONLY reason I am keeping the 4R. Of course, the 4x4 and Diff Lock are essential as well, but it is that V8 that saved it.