So what I'm wondering is, if I buy the game on PC, can I use the same login credentials on the PS3 client and start playing? Or do I have to buy another copy of the PS3 version in order to be able to use the account on the PS3? This is what I found from the official website:
It is possible to play with the same account on both Windows and PlayStation 3 platforms. Customers who own a copy of FFXIV for Windows and wish to use their account on the PlayStation 3 version will need to purchase a copy of the PlayStation 3 version. It will then be possible to use your characters on either platform by tying your existing service account to the PlayStation 3 version upon launching it for the first time. Similarly, customers who own a copy of FFXIV for PlayStation 3 and wish to use their account on the Windows version will need to purchase a copy of the Windows version. However, please note that your account will only be able to log into one platform at a time.
You need to buy both copies to play on either system. I tried playing on the PS3 and it wouldn't let me. (I own the PC version). Luckily my preorder is the PS3 version. You're "licensed" to play either or both "Final Fantasy: A Realm Reborn (PC)" or" Final Fantasy: A Realm Reborn (PS3)". You can check that in your account settings.