All I can say is make sure it's loud or you won't hear it if you're prperly asleep. If I was as worried as your wife I think the best device is your own ears and keep the kids in the same room so given what you're asking I'd say ditch the electronics and maintain the status quo.
And now for the advice you don't want (but you posted a potentially controversial topic on an open forum so live with it), get a grip, both ours were in their own cot in their own room from coming home from hospital. We used a normal baby monitor when they were very young and we were downstairs, upstairs with the bedroom doors open we were able to hear anything significant.
I also think some of the advice above is very pertinent, unless you know exactly how you're going to respond I think they are a dangerous waste of money. You need to learn to go with the flow more, for your sakes and the kids.
If I sound a bit off it's because my youngest has just come out of hospital this morning after being admitted on Sunday with a serious post surgery infection complicated by an existing condition. Don't wish ill health on them, take each day as it comes, they're too precious to waste, if your own kids get ill enough to need hospital treatment (and I sincerely hope they don't) maybe it will put this debate into context for you. If they have some underlying condition you haven't mentionned fair enough maybe they have some worth but if your kids are healthy be tahnkful for it.
