TJet from my perspective. TJet feels better balanced (not nose heavy like the diesel), it is $2k cheaper than the MJet and diesel is expensive here (95 RON PULP is average 159.4c/L in Perth vs diesel average of 177.2c/L). I don't do anywhere near the km's to make up the cost difference (about 8k km's a year). The only plus side of the MJet for me is the 6 speed box has a much nicer shift than the 5 speed of the TJet (mind you I drive a Pug, so I am used to crappy manual gearshifts).
The MJet sounds like a tractor, that is not the way a Fiat should sound. It has a narrow power band, while riding the torque is great for traffic, it does not seem very conducive to enthusiastic driving, where you like to give it the revs.
My 2c worth.
Troy.