I never used one - with any of my three.
I guarantee you will hear your baby when you need to - especially if you have baby in the same room as you for the first 6 months. In fact, there will be many a time when you get close to listen for breathing, even probably poking her/him (just to make sure), sometimes in the middle of the middle of the night in a panic! Sounds a bit crazy but definately completely normal, I still do it with my third!
I think its one of those things you think you have to buy when you are having a baby as its publisised on many lists of 'things to buy'. You say "got most things but still looking at monitors" as if its an essential item.
My reason for not getting one is that it has no relevance to the safety of a baby or preventing cot death or anything like that. I'm not aware that they can't choke on anything when little (unless you leave something small near them they can grab).
My concern is when you can't hear them, not when you can!!
The only case I can think of for having one (with a sensor) is if there is a medical condition and it is advised by a medical professional.. My friends baby had issues from birth with nasal/sinus and actually stopped breathing and they did get a sensor mat upon doctor advice.
If you are concerned about cot death (obviously we all are), follow the guidelines, don't smoke, don't overheat the room or put them in too much clothing/bedding etc, keep in your room for first 6 months (the CO2 you emit apparently helps with their breathing - I was told) plus whatever the leaflet says.
I would save your money.