I always get MMOs out of Steam, plus my FFXIV account is much older than my Steam account.

On the other end, if you pay with Paypal Steam makes it a lot easier than squeenix does. On mog you gotta buy crysta every time to buy anything, with paypal, and even if you have set auto sub renewal you need to add crysta every time before it renews so basically there's nothing automatic and it's pretty annoying. Buying stuff from the store too, every time you need to calculate how many crysta you need, add those before paying. With Steam you can just do all with Paypal directly without much fuss, so it's much easier.
Of course if you use credit cards instead that's not a problem for you.