I'd say, go ahead and get the large hide from reptile basics. Your girl will out grow the medium hide pretty quick.
I've got several of reptile basics' hides in several of the sizes and have nothing bad to say about them.
One other less expensive route: Go to walmart and see if they have the black tubs in their tub section. I've bought some of these and trimmed a couple inches off and made a cave shaped entrance. They work good and only cost about $2.