Actually, I have to disagree with you here. I owned an '04 SR5 V6 for a year and now I own an '05 SE V8 and my MPG between the two is VERY close. I think one of the reasons might be that the V8 doesn't have to work as hard, because it is very easy to keep it in the low rpm's and in turn save gas.
I was also concerned when shopping for a new 4Runner when it came to gas mileage, but when I compared the two, it only made sense to get the V8 because the price difference is minimal, plus the 4.7 has always been an excellent motor.
Yes, I would be happy with the V6, but I think if you have the choice and you don't mind paying just a little extra, the V8 is well worth it.