Re: 99 Durango??
I've never owned a Durango , but , I have owned plenty of Jeep and Mopar vehicles .
I had a 318 in my 89 W250 , TBI injection , manual 4 speed .
Anemic at highway speeds , even with a small popup camper you were losing speed / down shifting on the uphills .
Same 11MPG empty or loaded to the max .
Apples to oranges , I know ...
Still , those old "dirty engines" have a place in my heart , and , I believe the killing of the 4.0 is the WORST mistake DC has made recently , next to the pointles killing of the XJ , to sell the inferior Liberty , to then bring us the Commander . WTF ?
Yes I am a straight 6 fan!

They may not be designed for higher crankshaft speeds , but , for off idle through midrange torque , few other gas engines can match them within the same displacement range .
I'd love to see what they could have done on that same I-6 configuration , by a slightly shorter stroke , bigger bore , OHC , and , crossflow heads .
Sorry , getting off topic .
Get the dirty 5.9 , or , the newer "cleaner"

4.7 in the H.O. version .
Do keep the 5.7 in mind , but , I wouldn't pay a premium for it .
It ain't the legend who's name it was marketed in .
The most important considerations are that it has a factory tow package , or , aftermarket equivelants , and , that the brakes , suspension , and , steering are in top condition .
Next is the weight of the rig being towed , the tongue weight , and , the rated capacity of the tow rig .
After that , consider where you are towing , and , how far .
Fuel costs suck , but , so does doing 30 mph on uphill grades , with a herd of PO'd drivers behind you .
The smaller engine will be a PIA and deliver the same , or , worse , MPG as the bigger engine , especially in hill country , or , flat land with a big load .
Always get the bigger engine , if possible .
People never complain about the extra power , just the mileage .
Keep your foot off the skinny pedal unless you really NEED it , and , you will mostly solve that issue .
JMO