The Focus ST will never have over 400hp or run 12 second 1/4 mile times without some major cash outlay so actually the Mustang might be more practical in the long run if you're content with "only" 400hp.
I do agree that the Focus is probably more practical on a daily basis but if it's a 2nd car I'd take the Mustang.
'12 Focus Red Candy SE Sport 5 spd
'99 Toyota 4Runner Sport
'11 Subaru WRX STI Hatch