think about how much fun having a v8 in your toyota will be. the 3.4 will be more reliable, but the 5.0 is a much more common engine, if i ever have the $$ to spare i would want to do a 5.0 swap, all that torque of the v8 just makes me drool
unless you want to keep it toyota you might as well go for the v8 if your going to go through all the trouble to do the swap. i dont know about gas mileage and how that will come out but i never thought of my truck as an economical ride, but thats why i got a bike