Just get whichever has the cheapest total price of full install+obligatory contract. Then cancel the contract afterwards. That's assuming you live in the suburbs and know your neighbors, rather than out in the sticks.
Think about it- as soon as the siren goes off the thieves will be running...maybe even as soon as they hear the tone upon entering. The cops will show up 2 to 30 minutes later. What are they going to do...look around and say "yup...someone sure done did breaked in"? The noise gives you 95% of the benefit.