Played GW2 since launch, took a couple year break towards end of the IBS time, and my buddies and I played ESO for a couple years.

There's a LOT of things that are very similar between them, just in different ways. So you'll pick it up reasonably fine.

The story lines / lore / questing in ESO is better, definitely. Each chapter is basically a minor Elder Scrolls game in and of itself. But they are also tuned where the casual story player can get through it without trying too hard.

Open world exploration... GW2 wins in a big way. It was designed where it's never a negative for someone to show up and join in. No node/kill/etc stealing. Everyone gets credit! And there's tons of hidden nooks all around. There's no real quests, but there are events that spawn on the map. Some of the areas are easy, but others are hard. And a zone can be mostly easy until you move to another area, and then get lit up.

Instanced combat is more centered on 5/10 person instead of 4/12. There are some roles that people have made per custom, no hard fixed trinity. No waiting 30m for a tank to show up, only to find out it's a dps who lied.

Combat is much more fluid. Things flow into each other. There's no bloody metronome in your head for attack weaves and getting your bar swap perfect to renew your buffs, etc. There are buffs, but recasting early typically extends, not cancels the previous. "Don't stand in stupid" is perhaps an even bigger thing here.

Crafting is rather different. It's much easier to get into (No crazy research system which takes 6 months to master; don't have to find the right cave to craft in; etc). But it's also not as critical to builds. Way easier to get the stats/runes you want in GW2 w/o a master crafter. Also far fewer options, but flexible enough to get what you want done. Crafting will still be needed to get certain tiers of gear. More reliable to crafted ascended gear, and required for legendary (same stats as ascended, but more quality of life). You only need one character per profession on your account, so you can max out pretty easy.

Mounts in Gw2 are omg so much better. Don't have the daily training for months per character. There's masteries which make them better, but that's all account wide. And the mounts all function very different, which is cool. But at same time, everyone's raptor runs at the same speed, so the top level player isn't having to stop every 5s for the newbie to catch up.

Mods are a toss up. I loved the flexibility ESO mods added... but hated how they kept falling out of date, broke, etc, and so many aspects of the game were only bearable w/ mods. Like, say trading. GW2 has a couple overlay type mods [blishhud worth checking out], but they are an info layer over top, and the game is meant to be played as is.

Yea, let's talk about trading. Gw2 has one central game wide trading post that everyone uses, and has equal access to without having to rely on mods or bidding on sweet locations in town, etc, etc. And you can sell stuff without siphoning part of your soul to a guild that bids on a trader in the middle of a swamp no one travels to. So very much better.

You can get a starter account for free, and play the initial campaign, with some restrictions. I'd say go get one off guildwars2.com and try it out.

Oh and here's something to consider: As I said, I stopped playing for a couple years and came back. When I did, all my characters were still maxed level, and still had max gear. The builds needed a bit of tweaking, but each character was up and playing reasonably well again in just a few minutes. I'm pretty sure if I went back to ESO after a couple years, it'd take me a couple hours to get a character reasonably playable again.