There are pros and cons to both WoW and ESO but if I had to only pick one it would be Elder Scrolls Online
Pros of WoW...
I won't deny nostalgia I have for when I played it back in 2010 and despite the dated graphics WoW has great environments. The world feels large and they use color effectively that makes them pretty despite the dated graphics
That said 80% of WoW needs a visual update overhaul considering;
1. WoW is a top MMO in the market and makes Blizzard billions of dollars so they can clearly afford it
2. They have modeling technology as we have seen with the latest expansions
3. It has been over 8 years since Cataclysm updated the visuals
It's hard to appreciate the new models when old visuals of environments and armors cover it up
ESO beats WoW in quest design by a long shot.
Yes there is a lot of content but 80% of it is just outdated and poorly written.
I like reading quest text in games when it actually engages my mind and makes me think, but in Wow; The "get 5 rat tails" meme is no joke, there are lots and lots of stupid & unimaginative quests that will make your brain bleed even in recent content.
WoW has the some of the worst quests I have ever seen in AAA computer gaming. This is very important for a game that is supposed to be an MMO RPG. The Stories the player experiences along their journey is very important and if you can't regularly create a somewhat engaging story design then people are going to hate questing in your game.
Questing loses its novelty really fast in WoW, you're just clicking to get to the next one to level as quickly as possible.
I would equate Blizzard with a Bad Dungeon Master.
Their philosophy seems to be;
-have Great set pieces for the beginning so then players have good first impressions
-have Great set pieces for the end for a great & epic pay-off
BUT don't put any effort into the journey itself and fill it with meaningless tasks that are designed to extend played time instead of - consistently making engaging content and a fun experience.
It's no wonder Blizzard offers a level boost to new players because even they know that leveling is awful. Blizzard focuses on the end instead of the journey and it shows because, overall, questing feels like a chore instead of a fun experience. Leveling is a brain-dead ride while you have Netflix or a podcast on in the background and the fun doesn't start until endgame.
Maybe if the Developers focused on quality quest design instead of filler tasks they could actually have quests that are voice acted.
ESO, for the most part, has consistently engaging stories and side-quests. They made the right choice of making "Task" or "Chore" quest design relegated to daily activities, for the most part.
This four part video series explains in more detail what I am talking about and makes it easy to understand why WoW quest design is bad and what makes a quest "engaging".
https://www.youtube.com/watch?v=tWtvrPTbQ_c
https://www.youtube.com/watch?v=S5camMoNw-o
https://www.youtube.com/watch?v=otAkP5VjIv8
https://www.youtube.com/watch?v=ur6GQp5mCYs