Hi Tim , I think Lisas post was good, I would also take into consideration what exactly you want out of the cruise, if your daughter brings a friend you will have more responsiblity and exspence, but while actually on the ship you may have more time to yourself, mind you, unless you get 2 cabins , you will also have to be more careful about getting ready and sharing a bathroom. The cabins are small remember.
On shore you will have 2 girls to watch, but on the ship they will most likely never hang out with you(
) this may be a good thing, but ,if you want some serious bonding time with daughter then bringing a friend may interfere.
I think Lisas point regarding your childs personality is important, a shy child is not as likely to make a friend on the cruise, but many kids make friends very quickly on board.