The V8 will last a lifetime.... with normal maintnance
The V6 is going to struggle with the weight of the truck... and I was never too happy with Dodges V6 engines. If at all possible get the V8.....*slaps forhead* "I could'a had a V8"
You'll definitely want to avoid the V6; the truck it's trying to pull is so heavy that your foot will always be mashing the gas to keep up with traffic. As a result, your mileage would be about as bad as if you had a V8.
If the Durango you can get is new enough to have the 4.7L V8, from an power/economy standpoint that's the motor you'd want. It moves the Durango to 60 mph in the same amount of time as the 5.9 but gets a lot better mileage. The 5.9 is still a better choice for towing. The 4.7 is a newer design, though, so I'm not sure what the reliability is on that yet.