I agree with Richard that comp. insurance and off street parking are essential: I even pay to hire an off-street parking space for my car. As for Brant, Subaru's theft stastics would suggest their keypad immobilisers do a very good job (only thefts where after owners gave the key away due to threats of violence).
Either way, a top spec alarm should do you fine. Remember, its only ever a deterrent and nothing more.
LW.