For roleplay reasons it would definately be best if we could select it at character creation. It would feel very bizarre to try and roleplay as an already fallen paladin, complete with backstory all planned out in your head, to find you have to start as a normal paladin.
IIRC Oathbreaker isn't even in the PHB, but the DMG and not suggested for PC use. It is a nice little RP bonus thrown into the game that wasn't promised and I like that you have to fall to become fallen.
