A really loud alarm is going to scare most people off. We use a Monitronics system only because we knew the guy that sold it to us. I highly doubt that Brinks or ADT can be much better.
I can't give them a perfect score though, they originally installed the alarm siren in the attic and it could barely be heard outside or inside for that matter, and they call us in the middle of the night if the power goes out, they don't call the police but it's annoying to get a phonecall to tell you the power's out. We could probably get that fixed
You could add some motion detection lights yourself. if anything walks in front of them they'll light up the area. It'll spook someone wanting to go unseen.. and you could possibly rig it up to the doorbell so it digs when the light comes on... Sounds like a fun little DIY project