Both games are excellent examples of MMOs on console and cater to different markets giving Xbox fans a good amount of variety.
Both are very different games, so it really comes down to what you are looking for. If you want a Western-style fantasy RPG experience in an MMO world then The Elder Scrolls Online is the one for you, but if you are a JRPG fan then Final Fantasy 14 is a perfect example of the Eastern style in the form of an MMO. Both games are excellent examples of MMOs on console and cater to different markets giving Xbox fans a good amount of variety.