I would pick one of the faster S-Flex with a fan control if noise is a concern. Also Thermalright has come out with their own fan line (which are just re branded S-Flex's but vary in speed).
It looks like the Slipstream fans don't do as well when mounted on a radiator or HS.
Here and
here are some pretty good tests of various fans done by Vapor over at XS forums and well worth the read.
And I really like the
Sanyo Denki that I am using but they are hard to find. Petras has them but usually have been out of stock (were out yesterday but looks like are in right now).