The AC403 is the updated version of the much loved AC401.
It has updated software inside and a sleeker all white appearance.
Both of these monitors have 8 channels so should you experience any interference as is possible with any analogue/wireless product you have another 7 channels that you can select to reduce the interference.
As a result of the 8 channels you can use more than one AC401/AC403 or one of each model in the same home.
If you use more than one AC401 or AC403 we would also advise that you sticker each set with a different color or baby name, this will prevent any mix ups or queries should one monitor beep for any reason.