Looking newer

The standard trans. should help some with the MPG, if you can keep your foot out of it.

A GT will cost more on insurance and, unfortunately, needs to be considered as well in the budget.

If you stick with a 4.0 V6, some or all mods you have done may switch over so you can save some money there.

I like my 2012 V6, although I do have the Performance Package. The engine, which is rated at 305 hp, has plenty of power for either a causual or spirited drive. In a perfect world I would like to have a 5.0, but it was just out of my price range. When you consider the 3.7L has the same hp as the older 4.6L, you have a solid power plant to build upon. With the addittion of a 6 speed manual and up to 31 MPG, the 3.7L, in my mind, is a win-win!

But then you get those people that say "oh....its a V6? Why didn't you get a GT?"

Fuel economy is mostly about how fast you drive. With a manual transmission, you will be getting the best mpg on the highway at 55-60 mph. Unfortunately (depending how you look at it) most highways in our country have 65mph+ speed limits. At 55mph I can get 32mpg in my 2013 GT...at 70, that goes way down to 24 or lower.
If you have the budget, look into a used 5.0, the value of these cars drop significantly after 1 owner. For the price of a new V6, you could get a used 5.0. Get the 6 spd manual, shift at 2k rpms, and drive slower if you are concerned about fuel economy. You can still go full throttle at times...just don't be one of those people that constantly accelerate as fast as they can all the time. I still have fun in my car with WOT runs, and I average 25+ mpg.

Assuming you're young enough to have to deal with insane insurance rates, the V6 should save you quite a bit there. If you drive your car for what other people think, then the older GT might be a better choice. If you want good economy, good performance, and a much improved platform, I'd recommend an 11+ V6. Very impressive car, newer, so likely less maintenance issues, etc.