Do you need to pay for one desktop client (Mac/PC) if you have the other?

[DrWho2010]

you will need to pay for each expansion separately as well.

[DPZ2]

Well that's just ridiculous. Every other MMO that does cross-platform doesn't force you to pay for two clients. It's probably just an Obj C wrapper around the core C++ anyway.

Do you have to pay for each expansion also, or just the main client?

The expansion prices will have to be considered. Be aware that SE clients are the only true 'cross-platform' MMOs out there, where the term indicates players are not restricted to a particular set of realms.

The clients for Mac, PC and PS4/5 are definitely more than C++ wrappers.

There is no extra cost to play from two different PCs, although you cannot play on both at the same time.

[Packetdancer]

The Wineskin XIV uses was heavily customized by Transgaming in order to 'port' it to macOS, then Nvidia bought up TG and has been doing seemingly the bare minimum of upkeep since then.

It's worth noting that the lack of maintenance/updates on Transgaming's WINEskin was enough to prompt Square-Enix to dump Transgaming and move to Codeweavers' Crossover before Shadowbringers came out, and swap the client wrappers when Shadowbringers landed. So it's not quite so dire; Codeweavers is still actively developing Crossover -- including having made a fair number number of FFXIV-targeted optimizations for the Shadowbringers release -- so it's in a lot better shape than the old wrapper was.

(It still isn't a purely native MacOS-targeted client, of course. So there's that.)

[Rongway]

What about my second question - do I have to pay for just the main Mac client, or each expansion?

In anticipation of a possible followup to this: "What happens if I only buy the base game for Mac?"

If you buy the base game on Mac but not the current expansion, you can log into your character on Mac and play ARR and HW content, but only if you logged out of PC in an ARR or HW area. If you log out of PC in a ShB area, you will be locked out of the Mac client until you sign in on PC and move your character to an ARR or HW area.

If you purchase an expansion for Mac and then new expansions comes out, you will be able able to play up to the last expansion you purchased on Mac, and you'll be locked out of Mac if you log out of PC in one of the new expansion areas.