IIRC Garmin developed or was involved in the development of ant+. Polar didn't get onboard but developed their own version of Bluetooth 4.0. As a result the two brands focused on their own protocols. This meant, in the past, Garmin only used Bluetooth in the past. Polar is the opposite having.never used ant+ but users Bluetooth.
My advice is look at what you have and buy for that. For example if your bike computer is a Garmin get ant +. If it's polar get a Bluetooth sensor. Whatever you get a the Bluetooth can always be connected.to your smartphone.