I would advise getting the Vapor-x version for several reasons:
*According to the Sapphire FAQ on their site, the warranty covers overclocking to the limits of the Overdrive function in ATI CC, as confirmed by this link.
http://www.sapphiretech.com/presentation/support/?cid=2&psn=000202] (under "What is Overdrive?")
You will probably want to overclock the card more than that, but this limited overclocking is covered by the warranty.
*I have contacted Sapphire support and discovered that the
Vapor-x and Toxic cards are technically identical, with one (Toxic) simply having higher clocks set.
*The Vapor-X/Toxic cooler is confirmed by several reviews (sorry, no links, google 5850 Toxic reviews) to be quieter and cooler than the stock cooler by a considerably margin.
*The Vapor-X card can be found at £215 on Amazon if you're willing to wait for it to come into stock from official suppliers (currently listed as £260 from a third party)
However, the ASUS card offers easy voltage tweaking to obtain larger OC. I would still recommend the Sapphire card over that, as it does not require voltage changes to reach high clocks.