I've got a 2011 V6 Mustang and I love it.
However, I am a little surprised at how accepting everyone is of this 30+ mpg that Ford says the stang gets.
Let me be clear about one thing- the stock V6 does NOT get 5.1 0-60 at the same time it gets 30+ mpg. The stock V6 has a 2.73 rear end ratio as opposed to the track pack [or optional standalone stronger rear end ratio] of 3.31.
The track pack basically mimics the GT suspension and so has slightly better handling characteristics than the V6, but only someone who is really in tune with a rear wheel drive car will ever be able to tell the difference. And again, on the street, if you drive sanely you will never test the limits unless you are taking an on or off ramp at speed. The V6 suspension is that good [drove it on a racetrack up north here in Ontario].
I think if more people were to drive the V6, they would be more than pleasantly surprised with how far Ford has progressed with the engine.
I rented a Mazda 3 out west, [non turbo, lot less powerful], and with the front end tug on heavy acceleration with front wheel drive, the feel of the car is entirely different to the V6. The Acura CL V6 I traded in on the stang was the 260 hp model with about 240 lbft of torque and it had torque steer, even with traction control, so I imagine the Mazdaspeed will as well. Oh and by the way, I recently rented a Mustang GT with the ~ 400 hp engine, for a few days in Nova Scotia and brother, is that a powerful ride.
If you really are serious about either car, then for sure take a test drive and like mentioned elsewhere, don't forget about the WRX for comparison driving.
Good luck... both cars are nice in their own way.