From the way you described it, I can tell the salesman is trying to sell you a V8. if you do a lot of driving, those 1-2mpg will add up. V8's also depreciate at a faster rate. But, if you are gonna tow toys, don't drive too much or in rush hour traffic, go with the V8.
I went with the V6, because it is plenty. I tis my wife's daily and her trip computer shows 17.5-17.8mpg on average. I keep a log book, but I haven't calculated actual mileage, yet.
Also check the common issues and reliability ratings of both as well before you make a decision...
Regardless, they are both Toyota 4Runner's and are fantastic....