Of the two, I definitely go for the diesel & get it remapped.
I think you'd see a significant difference between the build quality of a GT compared to a MiTo. I love my GT to bits, but compared to the latest gen of Alfa's its not quite there as far as build is concerned. I've not yet driven a MiTo, but the drive on my GT is fine & at the moment I'm not thinking of changing it for anything else.
I do like the wheels on the cloverleaf model.
Pretty sure that the GT will be discontinued as a model in the near future, so values may well further drop. Get a well cared for, good spec model & if you're not worried about it being an "old" car, it should serve you well for years to come.