Raiding in FFXIV is different in the sense that bosses typically follow a specific rotation of skills they use. If you go watch some guides on bosses, you'll notice a trend of most video guides mentioning a long rotation of abilities. There are some exceptions most bosses follow this style.
In WoW, boss abilities are used more or less as they come off cooldown, and there is a lot of variance, like sometimes a boss might delay an attack for one reason or another and even though the cooldown based bosses might end up having a rotation of sorts, it's not like FFXIV where most bosses have very specific rotations they stick to.
In addition, raids in FFXIV are 8 man so there's a lot less room for error. If one person dies that is 12-13% of your raid dead. On WoW if one person dies you can likely just carry them. This isn't the case in FFXIV, so in that sense the content is less forgiving.
If you want my personal opinion on overall difficulty, WoW mythics are more difficult assuming equal skill raid groups.
And for my opinion on which is more fun, I'd have to say it's extremely close, but WoW very slightly edges out on top. I personally hate having to remember rotations for boss fights. Like, just look at the rotation for turn 9:
Phase 1 rotation Red dust > Beak > Beam combo > Yellow dust > Dynamo > Meteor Stream (x1) > Beak > Red dust > Meteor Stream (x2) > Beak > Yellow dust > Beam combo > Red dust > Beak > Dynamo > Meteor Stream (x1) > Beak > Yellow dust > Meteor Stream (x2)
First & Third Bahamut’s Favor rotation Claw > Fire (out) > Lunar Dynamo combo > Fire (in) > Fire (out) > Chariot/Beam combo > Fire (in) > Claw > Divebombs (first 2) > Meteor Stream 1 > Divebomb (final) > Meteor Stream 2 > Dalamud Dive on MT > a brief respite where Nael only auto-attacks before the next Favor
Second Bahamut’s Favor rotation (first row same as above) > Divebombs (first 2) > Iron Chariot > Divebomb (final) > Raven Dive to MT > Lunar Dynamo > that brief respite again
Remembering this shit is quite frankly stupid. I'd rather learn content by reacting to cooldowns than these rotations. But in the end, going in without memorizing the rotations and learning it by reacting is still doable, but you might run into issues where joining pugs and the party leader wants to "quiz" you on a boss rotation to make sure you know what you're doing. But really, in terms of fun both games have fun mechanics and interesting bosses, both games have their quirks and in my opinion WoW and FFXIV are the two best MMOs out there right now in terms of raid content. FFXIV also gets more frequent content patches, which is a bigger plus than anything else, imo.