As 74stang mentioned there is the sound.The 3.7l V6 does scream at high rpm, but there is nothing like that V8 rumble. More importantly though the 09 4.6 V8 and the 143.7 V6 have almost the same peak horsepower (310 for the V6 vs 300 for the V8) the 4.6 reaches peak hp at a lower RPM and typically makes more power throughout the power band. The 4.6 V8 also produces 50lbs/ft more torque that the V6 and again reaches peak output at a lower RPM. So if your asking which is faster stock for stock, all things being equal 09 GT is faster.
Also the '14 V6 is much more fuel efficient if you car about that.