definately depends on the intended purpose ... if it's gonna be a daily driver get the ST, toy only get the Mustang. I'd already be driving an ST if it weren't for the fact I already have 3 Mustangs for toys and a Focus Ti for DD/gas mileage. I've had 2 turbo4 cars in the past and both got only a hair better gas mileage than my 5.0L Mustang does which scares me off the ST a bit for a DD.
2012 Focus Ti Hatch, Blue Candy
2011 Mustang GT, Grabber Blue, 6spd
1978 Mustang II, 351W, Tremec 3550
1978 Mustang II, 2.3L Turbo, 5spd