We have the Angelcare one, recommended by a friend and recommended on again to another friend, out of the 3 I don't think anyone has had false alarms. It has a read-out where you can see when they are moving and it mutes completely unless they are making noise, so you aren't sitting listening to static all evening.
The way I see it you are buying peace of mind for your wife, and (especially with twins) the chance to get some well-deserved kip. My wife is a worrier in any case and we had some minor complications with ours which made it that bit worse. No matter how logical you are about it, she will sleep better with the monitors.