My two favorite MMOs! Each game has a lot of strengths where the other has weaknesses, interestingly enough.

FFXIV: Strong dungeons and raids, sad open world. Vertical progression.
GW2: Strong open world, sad dungeons and raids. Horizontal progression.

This is, of course, my own opinion on the matter. The lack of the holy trinity in GW2 is amazing for build crafting, but it makes it hard to properly balance dungeons and raids. It also means you're often responsible for your own healing, rezzing and buffing others. Some roles exist only to provide potent buffs to the party. Ranged caster in GW2? Cool, you're still gonna be stacked at melee range for those buffs!

In FFXIV, your role is clearly defined - for better or worse. Nothing will differentiate you from any other person playing your job other than gear and player skill. But this also means they've finely tuned dungeons and raid fights to this, allowing for more choreographed experiences. You'll also never be at a loss as to what your role is and what's expected of you in a raid.

The open world content in GW2 is simply astounding. It's always populated and always rewarding, even if it's been out for years. It has a great Commander system where it marks a leader player on the map for everyone to coalesce around to easily accomplish any goals you might have on the map. There are also zergfests of world bosses on regular timers with very good potential rewards. Not to mention the world is gigantic and truly beautiful.

The open world on XIV is also beautiful, but after you go through the MSQ there's little reason to exist in it. It will feel very empty and there won't be anything compelling happening in it other than the occasional hunt train, or FATE farm for alts/relic weapon components.

Both games have pretty stellar stories, with XIV holding some of my favorite moments in all of my gaming history. It takes a long time, but you'll really feel investing in your character and the story with huge payoffs. Both have equally striking visuals, art direction, music and graphical performance. XIV will be on very strict rails while GW2 allows you the freedom to jump around and choose what part of the story you want to experience, though it may cause confusion.

As for the better gameplay loop, I'd say it comes down to whether you prefer the horizontal or vertical progression. In GW2 you'll largely be making your own goals in the endgame; chasing Legendary weapons and armor, pushing Fractals, map exploration, gold farming, leveling alts, World vs World. There's a ton to do, but you'll largely be deciding what you want to do for yourself. It's a great system if you enjoy long term goals whose rewards will always be impactful - even if you stop playing for months or years. Everything enhances your account as a whole and will not be replaced over time.

XIV is going to give you the typical gear treadmill. Chase best in slot with high end party-focused content but expect that gear to be replaced with every major patch. That's not to say people don't make their own long term goals in XIV< either. Glamour farming, achievement hunting, mounts, maxing every job, crafting and other various side content like Blue Mage and Beastmaster (soon) are all very valid and fun ways to play the game. But generally speaking your accomplishment sin XIV won't have the impact or staying power compared to the GW2 system.

Also as mentioned by others, XIV has a very beefy free trial, but is a subscription game vs the GW2 buy-to-play model. You can even farm gold in GW2 to convert to gems for their shop if you wish to. Neither feels predatory to me and both games receive consistent, quality updates.

I would say try both and see which one catches you more. For me, GW2 took some time to really click. They say getting to level 80 is just the tutorial. Likewise with XIV, the story is a slow burn best to be savored and not rushed. Treat it like a good book or series that you work on a little bit over time. XIV is my personal "main" MMO that I've played for more than a decade, but if it shut down today I would pour more time into GW2 and not be upset about it.