Ethernet is always better if you have a choice. Wifi is always subject to random interference from signals both inside and outside your house.
Wifi work pretty well for watching movies if you have a good signal. It can tolerate small issues with interference because it uses a buffer to hide data retranmissions. Large issues would cause a stall/buffering in the video.
Pretty much wifi is acceptable for most application other than online games which are affected much more by data retransmission in wifi.