You could play your character on both platforms so long as you're in ARR areas. You wouldn't be able to access expansion regions or expansion quests while on PC. And I believe you would only get XP on jobs that are below level 50 while playing on PC, though I haven't tested that to be certain.

The lack of expansion region access also means you couldn't log on from your PC if you had last logged out in an expansion region. (There's no feature to automatically port you to a region you can access.)