Can't decide between Ecodiesel or Cummins

This is my first post on here, so I apologize if this is in the wrong location.

I am currently driving an 08 Silverado 1500 with the 5.3l, and I want something that pulls a bit better. I pull a 16' tandem flat trailer about once or twice a month for a three hour round trip with mostly interstate driving. I do not know how heavy it is loaded at the most, but my Silverado feels the trailer on acceleration. It can be a pain to accelerate on the interstate if I need to, and instead of getting a truck with a larger gas motor, I am thinking a diesel. I want to know if the Ecodiesel will be a noticeable difference over the 5.3, or if I should go straight for the Cummins. I am loving the mpg figures people are throwing about for the Ecodiesel, and this truck is mainly a daily driver, but that Cummins power though...

Also, in both the 1500 and 2500 crew cab, the back seats fold up for storage space. Do both models have the flat folding floor under the seat in every trim level? It is a necessity for me. I tend to throw stuff in the back seat in the winter that cannot get cold, and it requires a large flat area.