On the mpg front, an important thing to note is that the 6.4L Hemi is able to switch into 4 cylinder mode when running empty, the 5.7L in the 2500 doesn't have that ability as it is not equipped with MDS. RAM engineers say the 6.4L Hemi will therefore get better mpg than the 5.7L (Video).
X2 Everything I've seen has the 6.4 getting better gas mileage than the 5.7. I imagine mostly due to MDS. Not sure why the 5.7 doesn't have it since it is available in the 1500 series.
Overall, the 6.4 seems to be the better buy. That being said, they are still pretty new and hard to find on dealer lots. I couldn't wait the time it would take to order one so I ended up with a slightly used 2500 with the 5.7. Haven't towed with it yet but it has the same power as my outgoing Tundra which hauled my 8k tt just fine. Mileage is worse, averaging 14mpg, but then it curbs 1k more than the Tundra.
