Direct Connection vs Wireless
The TV will only use data on its own if it's set to update or refresh its apps and such automatically, so you should check to ensure that whatever apps you use on the TV aren't set to use data automatically. It's not dependant on Ethernet vs WiFi. Also, the one that might actually use more data in practice is the WiFi connection, but pretty much only if the WiFi connection is weak, as it may need to resend data packets to transmit all of the data. If the WiFi connection is strong, you'll use the same amount of data either way.
An ethernet cable can transmit data faster than most WiFi, and therefore more data during a given time, but that's only its capability, not necessarily what it would actually do in practice.
Technically, an ethernet cable is going to always provide the widest bandwidth, therefore the greatest thruput, which will always seem faster. Wifi depends upon the protocol used: 802.11ac when using multiple 20MHz channels (up 80MHz wide) will approximate ethernet thruput, whereas 802.11a, b, g using a single channel will not.
Data is data and neither uses more than the other, however a wider/faster path will always use data *faster* so people tend to think it's using more data.
