Personally, I stick with Dot 4. No compatibility/contamination problems and as I change it every 2 or 3 years water absorption isn't an issue.
I'm quite happy to accept that Dot 5 may be better in extremis but I've never had any problem with Dot 4 and I'm a fairly enthusiastic driver.
I changed my brake fluid earlier this year, I bought a new 4 litre container of fluid and a tester to check whether the old fluid showed any sign of contamination. Not a sign, and I tried it on an already opened bottle of fluid (although the cap was on tight) left over from the last time I flushed the brake lines. No sign of any degradation.
