I'd say Orochimaru, he's smarter and has more tricks up his sleeve. In close quarter, they'd be even. Samehada can't overcome the Sword of Kusanagi, but Kisame's brute strength alone would make up for it. Both can take hits very well, but Orochimaru is harder to kill. Manda will beat large-scale water attacks, including the water dome. Kisame's shark form is a gimmick within water dome, so I wouldn't say it's significant. Orochimaru would definitely win, but it would take a great deal of effort.