For me, I would go with the Hemi if I drove less than 15k miles a year. You are only looking at roughly $200-300 fuel savings a year with the Ecodiesel and roughly $50-75 of that is eaten up due to the expensive oil and filters for the Ecodiesel. I would gladly pay that for the added performance from the 155hp extra horsepower the Hemi has.
Now if the Ecodiesel had considerably more pulling power than the Hemi like the Cummins has over the Hemi in the HD trucks, then I would go with the Ecodiesel. However, the whole premise for the Ecodiesel is to be ECOnomical(not more powerful) and the savings is just not enough for the amount of power you will sacrifice for most people who drive less than 15k a year and do not tow very often. Heck, for that small of a trailer, you would probably save more money going with the 3.6L if you don't tow very often.
OP, do the math on total cost of ownership. You may or may not come to the same conclusion.