I want to imply one is worse then the other.
I friggin' hate it when i play some online game, put some time in it, do well, and then get destroyed by some kid who bought all kinds of extra items. I don know many games on facebook, but the ones i do know give people who do not pay such a disadvantage, it's not funny anymore.
It's even worse when the things you can buy with real money change after a few months. Giving the people with online money a bigger advantage.