We have used both and they are both good. We only use the snuza now though as when DD comes into our bed only the snuza will work at monitoring her breathing, as the Angelcare pads would pick up our breathing too if it was under our mattress. The snuza is more versatile as it can be clipped on wherever bub is sleeping.
DD is 10 months and we have never had to change the battery. She's been wearing it every night since she was about 2 months old, so I think the battery life is variable. We did buy a spare but it was very cheap.
Only thing with the snuza is we do get the odd false alarm when DD rolls around in bed andit comes off the nappy. It doesn't happen often though.