Yes, you can use an Ethernet cable, as long as your laptop has an Ethernet port (some newer ones don't).
With regard to the speed, it depends. An Ethernet connection will almost always work at full speed, whereas the WiFi may need some setting adjustments to do the same. If those setting adjustments are made (if needed), a WiFi connection can be equal to an Ethernet connection. Mine is. I get the same speed with either connection.