What IOS will be on my phone if I restore to factory settings?

I just got an iPhone 5 to replace my old 3GS, and I want to restore my old 3GS to factory settings either sell it or give it to a family member. (I'm hoping that when I do the reset it also will improve the performance, because the battery life on it is awful.) Does anyone know what iOS version will be restored to the phone if I do a factory reset? It's got iOS 5.x on it now, and I wouldn't want it to go back to iOS 3.x (or whatever was originally on the 3GS), but I'm worried that if it does go back to 3.x the only thing I'd be able to update to is iOS 6, which I'm sure is not going to work well on the 3GS. Anyone know? Thanks.