CB:
I got the limited V6. Its strong enough. I had 2005 Mustang GT that I traded for the 07 ST, and stepping down in power and up in weight was easily noticed (I also had an '04 Mustang MACH 1, '04 Mercury Marauder, '03 Mercury Marauder, and an '02 Mustang GT). But, with the motor breaking in now at about 2600 miles it has enough pep. My only critique is that the 5 speed takes too long to hunt for gears when you push the pedal to the floor. Towing and payload capacity is hight than the 99 F-150 V6 that I had. I'm guesing that the 3.73 gear (that I believe the '07 ST has) helps with this. Think about it: my '89 Mustang 5.0 LX was only rated at 225 crank HP. That was BIG power in '89.
Bottom line: The 292 HP in the 4.6 3v makes the Trac scoot much quicker. If you want to get through the 1/4 mile in half a day instead of three quarters of a day, or tow a deep V fishing boat, get the 4.6. For its intended purpose, the 4.0 doeas a good job.