Both are excellent remarkable titles. Personally I enjoy playing Shift quite a bit more. Overall presentation is fantastic and just extremely fun. Nova will keep you playing if you enjoy playing death matches. Shift will keep you coming back for its content (stars, achievements, cars+customization), local multiplayer, and overall an awesomely fun experience.
Both are good, but I prefer N.O.V.A. over Shift because although there's more customization in Shift, I still think the AI in Real Racing is better, and I don't think Shift is that much better in graphics and presentation. N.O.V.A. is by itself, because there's no other game in the futuristic FPS genre that has single-player gameplay. The only thing that can come close is Modern Combat, but you didn't mention it, and N.O.V.A. is superior to it.
Thank you Craig Couldn't have said it better myself If you run polls like this, the games should really be in the same genre...
