I'm planning on installing 3/4 T&G red oak on the 2nd floor. This would be my first time installing a hardwood floor. I've already bought the other tools I need. I've decided to buy a manual nailer and can't decide whether I should get the Powernail 45 (singe hit) or the Powernail 45R (ratchet - multi hit) nailer.

Why not rent a pneumatic and make life easier on yourself? Or buy a pneumatic and sell it on Ebay after you're done. Many others do.