Actually I'd go with both! Better safe than sorry...Car alarms do tend to deter people from taking cars from most statistics. The one statistic that matters is if it lowers your insurance, in my book. My insurance is considerably less with a passive thief deterant system (what most cars come with now) In fact, in most cases most models I've seen seem to come with the cars. Maybe not in your area. However they aren't that expensive anymore and many electronics places will install them for a nominal price.(Sometimes included in the price of the alarm!)
One feature I do love about my alarm, though...If I press the "lock" button on the remote keychain, it honks. That lets everyone know I locked my car. :agree:
My feeling is that it can't hurt to get it. Right after we brought my 2001Toyota Corolla, we stopped to buy a brand new "club" on the way home. A week later we arranged to have the security system installed. I'm glad to have both - a visible reminder to any would-be-thief, and the internal one. So far, so good, and of course, I hope it stays that way.