You have completed 0 (zero) miles driving 'your' MiTo since what, 2 months ago?
A diseasel, big or small, will return similar mpg on short trips compared to a V6 and
will have all the issues associated with DPFs, EGRs, regens, higher servicing costs etc etc.
If you just count the pennies on what's going in the tank you are only fooling yourself
FA57 GTA is selling a gorgeous GTA, it's in excellent condition with lots of goodies, you could drive it for 2 years and get rid for what you paid. It'd only do 21
36 mpg with you driving it but overall you would be better off.
Oh, and remember....
...this is it. Life, one shot. Go get.