GW2 is more populated but it has spammy combat with rather shallow depth-curve despite community hype. Tera is less populated but really perfected their action-combat and had some astoundingly good netcode.
Tera has significantly better graphics quality than GW2 in every aspect (environment, armor textures, character models and textures). GW2 is still a nice looking game though. It's just not remotely on Tera's level visually.
GW2 is more western in its art-style and delivery, Tera is a mix of western and eastern styles.
Tera's BG's were not very well implemented and need to be tweaked to be more interesting, while GW2 has more thought out battlegrounds system but because of the fairly shallow curve I referred to earlier, I don't find them very fun. The act of playing my character in GW2 just isn't horribly intriguing.
Tera is much more of a time-sink because of its crafting and enchanting systems, while GW2 is a very casual game with a very casual community trying to project it as an E-Sports quality pvp scene such as Season 7 WoW (which it is not even remotely close to in its current state).
So which game is better? If you want to enjoy an MMO for a lot of technical achievements and a strictly cosmetic frivolous cash-shop that does not impact the day-to-day function of your character, try Tera. If you want to be a part of the game of the month that doesn't do anything incredibly well, but doesn't do many things horribly wrong, but DOES force the community toward using its cash shop (gold is heavily restricted in GW2 so don't believe the hype about being able to buy anything from the cash shop with in-game gold as well because it just isn't a reality), go to GW2.
Personally, I'm going to keep giving GW2 a chance since I've finished the hard mode content for Tera currently, but I had a lot more fun in Tera than I'm having in GW2 so far, and the way that GW2 is strangling gold-generation in game to try to force people into using real money to buy things like bank slots (which you get very few of otherwise) is grating my nerves. GW2 feels like a very cash-grabby flavor of the month game and I'm pretty disappointed with my purchase if I can be completely honest.