Angel Care Monitor most definately for the first bub.
I bought one for my 3rd baby and I have never regreted it once, we had a family member loose their baby to sids so I had to buy one just for peace of mind. I still use it and he is 14 months old, we have had a few false alarms which does scare the crap out of you but at least you know that it is doing it's job. I have only great things to say about it, it is worth the money as mine has 2 pads on it for the upper and lower end of the cot.
I purchased mine from toyworld but you can get them all most anywhere now and probably cheaper. My little man has a cold at the moment so it's great as he is so snuffly with his breathing that I am happy to leave him in his own room with the monitor on.
Hope you decide on the right one for you!