All in all, honestly alarms won't prevent theft. The difference between a smart ($$$) and dumb ($) alarm is that the "smart" alarm will take longer to decode or discramble. Meaning, let's say if it takes 30 seconds to decode a dumb alarm, it might take 5 minutes to decode the smart one........ and in those 5 minutes hopefully someone will see the thief and deter him.
If i had the money, I'd put LoJack since it will actually get my car back in case of theft. However, I've heard that LoJack turn around averages 2-3 hours per car......... and in that time a professional thief can pretty much strip my Civic to its chasis.
The best combination would be a smart alarm, LoJack, and some flamethrowers (N/A in the U.S.)
More about the latter here:
http://www.gettingit.com/article/69