How should x264 presets be used?

[pwnsweet]

I have a grainy film (The Guardian) that I'd like to encode. x264 has both 'grain' and 'film' presets, so which preset should I use? Can I use both? If not, what effect will using either preset have?

[Dark Shikari]

--grain is a tune intended for uniform grain preservation at high(er) bitrates. It's basically a more extreme version of film.