Former Fit Owner. It was a 2007 base with a 5-speed manual. My best average MPG on a plain old week of going to work was 33. The TL has Honda's 3.6 (I think) V6 mated to a 5-speed auto, most likely. Unless it's the S-Type, which has the 3.8 and a 6-speed. Either way, you're going to be FAR more comfortable in the TL, which I think is maybe the most comfortable car Honda has ever made. Those seats are amazing. Assuming you don't have the explodey transmission, which I think was resolved in late 2006, your car should be good for a least a quarter million miles with no major work. Knock on wood.
At the moment subcompact resale values are in the toilet, thanks to cheap gas (there's a rumor going 'round that Honda is losing so much money on the new 3rg gen Fit that they might drop it from the US market in 2019 in favor of a Civic hatch). So while you COULD get a 2nd gen Fit for not much money, it's still an expenditure that would take YEARS of driving to recoup in fuel savings. As someone mentioned, you can run a spreadsheet. But in my opinion you will be far less comfortable in the Fit, and if you drive much at all, that's an intangible that will nevertheless impact you (and your back) for as long as you own it.
I'd keep the TL.