I leveled a character in ESO to max, and got into the first few veteran levels before they introduced the champion system. I have 2 level 65's in TERA and have spent the past few months solely in this game. Here are my impressions:
ESO
Pros
Excellent voice acting. Literally everything everywhere is voice-acted. It really does help with the immersion into the story.
The Elder Scrolls lore. If you enjoyed Oblivion and Skyrim, there will be things in ESO that make you happy.
The new justice system, which adds some interesting PvP opportunities to the game. I haven't dabbled in that yet.
The story lines you play are often times very interesting. DO NOT skip through the quest text / voice acting. This is seriously the best part of the game.
Cons
People don't talk. One of the best aspects of an MMO is the vibrant community. ESO is like a single-player game a lot of the time.
Limited character customization; very little "fashion" or abilities to really make your character stand out
The combat, while massively improved from launch, is still "meh".
The skill animations and skill customizations are limited and not very impressive, especially considering the Elder Scrolls heritage
You're stuck picking from a small number of skills you can use at any one time. Which means two players who play the same class probably use the exact same skills in the exact same rotations. Everyone becomes a carbon copy of everyone else playing the class.
Everyone wanted Skyrim Online, and set expectations accordingly. The game sucks balls compared to that ideal. Music is meh, graphics are outdated, animations are not smooth, there are no monsters that really make you feel "afraid" like the first dragon you run into in Skyrim did.
TERA
Pros
The community is filled with social people. Global chat gets crazy sometimes, but that's part of the MMO experience.
Combat is fluid, animations are impressive, skills look fantastic.
Graphics are generally high-quality.
There is true open-world PvP, unlike most modern MMOs where PvP is limited to "world versus world" instances
The story lines and quests are often comedy gold. Reddit memes show up all over the place. It's obvious that the game authors have a sense of humor, and it keeps the game "light-hearted".
The opportunities for character customization are endless. You want to be a giant panda bear with a carrot as a weapon who rides a modified pig with a Harley Davidson engine? Go for it.
Cons
The dungeons are fairly standard. They're fun, but they can get repetitive. After playing through some of the dungeons in Diablo 3, I'm a firm believer that MMOs need to adopt some kind of randomization to shake things up and keep players engaged. Just my opinion.
The ability to specialize your class is limited. Some games allow you to take your class in all sorts of weird directions, specializing a fighter to sing bard songs and shoot lightning. Every player in TERA will eventually earn all of the skills available to their class. There are a lot of skills, so different players might use the same class in different ways, but there's nothing like the "multi-classing" options available in games like DDO.
Items in the cash shop are expensive. Not that you NEED any of them to play, but if you want a cool mount to ride then plan on dropping $15-$20 in the game. Same for costumes, weapons skins, etc.
The PvP can be good, but if you've ever played ArcheAge and been involved in ship battles then this game is a little lacking.
Overall, I think TERA is more fun and in spite of the better story ESO never really got me "addicted". TERA is a very fun game that's currently going through a sort of renaissance and gaining a lot of new players. ESO's population has been on steady decline since launch.
All of this is purely my opinion, but I hope it helps!