"I can potentially see some confusion for users"...."for launch we are only going to spend time on one" I think you've answered your own question here. I'd go

"I can potentially see some confusion for users"...."for launch we are only going to spend time on one" I think you've answered your own question here. I'd go for option A - it's something users will be more familiar with from their experience using other sites and it stops you losing users who don't want to create a google account if they don't already have one. Your users are what is most important and the deciding factor in decisions such as this should always be to do whatever makes things easiest for them. Also, why do you need to link a user to an email address to handle payments?