I'm thinking about buying one, but I can't decide on which. I play rock music and I'm looking to get a watery sound with some sustain added in the mix.
Can't think of a song with chorus?

Paradise City?

TS, For the water, liquidy sustain sound you're looking at Chorus, all the way.
Chorus is more subtle, I never really pay attention to it when I hear it. It's hard for evh's flanger not to punch you in the head.
I don't give a shit if you listen to me or not
Go to the Musician's Friend website and have a look at the Boss pedals. Most of them have a sample audio file you can click on to hear what the pedal sounds like.

Quickly though, Chorus is an effect that sounds like multiple guitars. Flanging is the sound you achieve when you spin two records in-synch, then let one get slightly out-of-synch. It's a very cool effect and club DJs use it. I'm trying to think of some songs that use it, but I'm drawing a huge blank.