Being the creator of OpenQA and Selenium RC, my bias obviously leans towards considering Selenium a solid choice. Our recent release of 1.0 beta 2 brings us closer to a final 1.0 version.
That being said, Watir/FireWatir is also a reliable option. While Adam is correct in noting that WebDriver will eventually merge with Selenium to form Selenium 2.0, he is mistaken in suggesting that Watir does not utilize native hooks. If Watir were just a duplicate of Selenium using JavaScript injection, I would not recommend it.
However, because Watir uses native hooks, it has capabilities that Selenium currently lacks. Although it may support fewer browsers, its functionality in the main supported browser (IE) goes deeper and allows for control beyond just the page itself.
In conclusion: both options are viable, Selenium remains a strong contender, and sticking with Selenium for a bit longer will ultimately provide you with the best features from both worlds through WebDriver/Selenium 2.0.