From the sound of the original poster I get the feeling power and modding isn't in the cards. So with that said as for reliability the the two are the same. For fuel costs yes the GTP requires premium but given the size of the tank it's roughly $2 more then regular (different locations may vary) MPG are roughly the same.

From what I gather from your original post I would buy the best car for the money as far as mileage/cost and options.

I'm betting you'll be perfectly happy with an optioned out GT as well as a GTP.

I do recommend getting one with all the bells and whistles. I don't know how many posts I've seen from SE/GT guys asking how hard is it to add "X" option that OEM.