I agree with Bear II in that I don't worry about the money I spend for gas, I travel until what I have budgeted is gone and wait to accumulate more.
My figures may not be relevant and the amounts may be extreme but;
If traveling 10,000 miles pulling one trailer and average 16 mpg in comparison to the pulling the other trailer and getting 14.5 MPG, in the end assuming $3.00 per gallon cost, you will lose $0.01 per mile pulling the second type.
Of course there are the variables of purchase price differences and comfort and ease of driving, assuming all things being equal, $0.01 cents per mile isn't much. After driving 10K miles the difference is the 16 MPG trailer will get to go an additional distance of 90.50 miles.
I think I did the calculations correctly but maybe I did it wrong but if my figures are correct, what was the question again?