The Impreza will probably work a little better b/c of the AWD, but it will be more expensive to buy and run. The Golf has the advantage of being dirt cheap (no pun intended) to buy and maintain, and it has a ton of racing and rally support. It's also much lighter than the Subie.
In beginning rally or rallycross, durablility and ease of maintenance is far and away the most important thing to consider. Don't worry about absolute performance. Finishing the event is more important.
Good luck!