im not positive on this but ill answer anyways.
a physical affair is when someone likes the sex, and such things with that alone. you dont have to have sex with another for this, but quite often it has.
a emotional affair is one with feelings for the other person ie. love keep in mind you dont actually have to have sex with the person with this affair.
so more or less one is lust the other love.
i suppose in my opinion a emotional affair is worse, because a physical affair is only for sex which means they usually have no real feelings of love towards the newcomer. in a emotional affair it usually means the person loves/feels drawn towards the newcomer, which puts a even more serious damper on the current relationship.
personally if my girlfriend ever cheated on me i would end the relationship no matter what type, but everyones different.