Twin or Full?

We got the old bed and furniture out of what will become Sidney's big sister room.  I'm scouring the internet for deals on beds.  The problem I'm running into is what size bed should she get?  I like the twin because it's smaller which allows for more space in her room.  I like the full because eventually her brother will have a full size bed because that's what the crib folds out into.  I know it'll be awhile until he's in that size, but I don't want her to be upset her little brother has a bigger bed than her.  I had a full size growing up and Jason had a twin.  I don't know that he ever complained.  What would you choose for a little one?