From a money standpoint, look at it this way: The E rated tire weighs about 49 lbs. according to specs. The C rated tire is 43 lbs. That's a 6 lbs per corner difference in weight if the specs are telling the truth.
That's not going to be a major factor at the fuel pump. You're talking maybe 0.5 MPG difference, if that. So the money you save on fuel by going C will take probably 18 months or so to pay for the extra price difference. After 18 months you'd start saving the cash. I mean, we're talking 6 or 7 dollars per month for the avg. driver at current gas prices.
From a ride quality standpoint I think the E will be stiffer of course but the Duratrac is not a supremely comfortable tire to start with so it's not a big deal at all in my opinion.
Check out discounttiredirect.com before you buy and make sure your quote is reasonable.