One thing I would look for is a system w/ a 'real time response' or 2-way link, meaning that once the alarm is triggered, and it seizes the phone line to call the monitoring center, like others, but then the center then can create a 2-way link over that line, and you can talk to them real time, w/out having to get to/answer the phone to talk to them.
Here are the benefits of that to me personally:
In the event of a fire or a burglary when we're not at home, they can actually hear what's going on and relay that info to the first responders.
Particularly in the burglary, they could hear someone in the house, and relay that it is in progress, as opposed to a blind sensor indicating the alarm was tripped. Theoretically, some police stations could interperet this as a higher level priority, as the alarm company has confirmation someone is in the house (not the homeowner w/ the correct code), rather than just hearing an unanswered ringing phone w/ no way to confirm an unauthorized person was in the home.
Additionally, we've tested ours, and they can hear things you say, even when you're talking quietly and someone in the next room couldn't hear you, at least that's the way it is w/ ours. So you could pass on info to them, potentially w/out the intruder hearing you. Or, if you're hurt and can't get to the phone, you could still talk to them over the 2way intercom link they create.
Some have concern over big brother type thing, but I have a monitor on the phone line which can tell if a line is seized, and I've never had an issue, and they claim it can't be seized from their end, we have to manually activate it, or the alarm has to go off, in order to call the monitoring center and create the link.
We have a preschooler, and occasionally have elderly people stay with us, if we were out, or unable to get to the phone, unconcious, or what not, the smaller children or the elderly may not get to the phone, or not choose to for whatever reason, but could more easily communicate w/ the center over the two way w/out any special training required.
Theoretically, again, if an intruder was in the house, and is unable to give the monitoring center what they want, and in addition to the siren going off, they have someone talking in their ear that the police are on their way, as opposed to a ringing phone in the background, where the intruder knows it may not be a high priority call, and he may still have some time, etc.
If in the home, you can either get to one of the panels, or opt to get the key fobs, and activate from anywhere, and talk to them over the 2 way.
Another thing to consider, is either ensuring your phone lines are terminated inside the house, like inside the garage, making it more difficult to cut them, or have a wireless back up on the unit, depending on how important it is to you.
I think the other posters above offer some good points to think about, to get a good deal and save money, and have the option of various companies offer the support/service later. However, at the end of the day, make sure it's going to do what you are intending it to do, when it really counts.
I would want to know what their response time is like, some monitoring centers basically suck, and when it really counts, that could be bad. Again, I included fire w/ mine, and don't understand why anyone would consider getting a system w/out getting fire monitored. IMO, there's as much, or more chance, that this would get used before the security, YMMV, hopefully neither get used, but you know what I'm saying.
Also, when it comes to the equipment, especially if you go w/ a more traditional company, as opposed to a DIY deal (although maybe they offer this too, I don't know), I'd ask about a false alarm guarantee. We had an alarm on an older house that was acquired by inlaws before I was in the picture. It went off so much that they stopped using it, because they would get fined by the city if they had another false alarm. To fix it, it would cost an unknown amount of money, probably have to start over, coz the monitoring company seemed unable to fix it, unless they started swapping parts. Our company offers a false alarm guarantee, if the alarm goes off as a result of faulty equipment, they'll pay the fine.
This leads to the next point, future cost to repair. Again, w/ a traditional company, ask them if they'll replace defective equipment on their dime. If not, I'd consider that when obtaining the initial deal, ask how much the components are separately, if you have to replace them on your own.
For us, I looked at the cost of one company that was much cheaper on monitoring, but for all the sensors I wanted, I had to pay more up front with them, and pay for any parts if needed a replacement. The other company, the monitoring was more, but I got the 2 way link, paid less up front, and they replace broken equipment free, no trip charge, nothing, and offer the false alarm guarantee. Over the course of the term, the cost ended up being almost the same, but I got more w/ the one w/ the higher monitoring, w/ less out of pocket up front.
After the end of the term, I can have someone else monitor it, or renegotiate the term w/ this company, but you gotta ask all that up front, to know.
Hope that gives you some things to think about to find what you want. Our company only operates in the Carolinas, so that may not help you. I do know of one national company that does the 2 way, but not all do it. Ask around...
Karz