Get a pressure pad one, because if something happens for the sake of a few extra quid and a clicking noise you'll regret it for the rest of your life.
Our first baby died at 4 days old, during the night.
For our subsequent children we had a pressure pad monitor (a Johnsons then an Angelcare). Our second had one instance of tripping the alarm, we went to her and she wasn't breathing, the alarm was set to 20 seconds so by the time we got to the cot she hadn't breathed for almost a minute. Jolting her out got her breathing again but she spent the rest of the night in hospital being checked over.
Whether she would have started again on her own, probably, but the alternative was unthinkable in that stage of our lives. It paid for itself a thousand times over in one instant.