Obviously since it's a focus forum, I'm going to say get the ST. It's more practical than the mustang by far. Not only in terms of fuel consumption but cargo space as well. I remember having to ride in the back seat of my friend's mustang with a 40" TV on my lap because the trunk wasn't large enough to hold it. My favorite part about the ST though is the fact that it feels like a boyish hot rod while still being refined and practical enough to be driven every day.
At 108 MPH i can still get 27 MPG.