Since the other car is AWD (big fan here) and a versatile hatchback, the Mustang would be a hard sell if it were me.
I don't know how much longer the life cycle is on the WRX, but the Mustang is getting a major change for 2014, so now would be the worst time to buy one unless you're totally in love with the current one and know you won't like the new one. There's a lot of different mock-ups of what it may look like, but one way or the other, 2014 keeps popping up as the year it happens. The pictures I've seen suggest strong influence from the Camaro and G37, along with the new Ford Aston-Martin-ish grille.
They're great cars, but I think they'll be greater next year, so if you end up getting your heart set on one, I'd soldier on another year with the VW. Again, just an opinion.