It is worth noting that any Ethernet cable should be max length 100 metres. If a cable run is longer than this, then you will start to lose quality of service on the network.
It depends on lots of things - is it an open-plan office? Are there any walls that might affect the signal strength? You could even do a mix, have one on WiFi that's furthest away, and the other two on ethernet into the router - ethernet is generally faster and stronger, and more reliable, than WiFi in my opinion...so I'd always do Ethernet where possible.