Killer 7 is more of a rental, but chances are you'll keep it anyway (like me) just to figure out the oh-so confusing story. Suda 51 (the guys who did the story) only told a few things, and left the rest to the players, which I believe they are still trying to figure out.
Geist, I'd say is more of a buy, but if you're not sure, I'd rent it. It sounded really innovative when I got it, and I hadn't played any good FPS/FPA hybrids since the Metroid Prime series (more focus on the FPS though). The multi is really good, and one of the few handful of shooters out there that allows bots. It works VERY different from any traditional sense. There is ammo, but you get it from a reload station, and you don't pick weapons up, but rather possess the host who has the weapon.