If you're looking to make a serious long-term investment in a framework that will continue to be improved and supported by the community, Selenium is probably your best bet. For example, I just came across this info on Matt Raible's blog:
  As of Friday, Google has over 50 teams
  running over 51K tests per day on
  internal Selenium Farm. 96% of these
  tests are handled by Selenium RC and
  the Farm machines correctly. The other
  4% are partly due to RC bugs, partly
  to test errors, but isolating the
  cause can be difficult. Selenium has
  been adopted as the primary technology
  for functional testing of web
  applications within Google. That's the
  good news.
I also went to one of the Selenium meetups recently and learned that Google is putting serious resources into improving Selenium and integrating it with WebDriver, which is an automated testing tool developed by Simon Stewart. One of the major advantages of WebDriver is that it controls the browser itself rather than running inside the browser as a Javascript application, which means that major stumbling blocks like the "same origin" problem will no longer be an issue.