in my opinion, neither!
Alexa is especially unreliable. It's data is often screwed up. For example, it once reported that youtube had surpassed myspace in pageviews where as a reliable data company had said for the same month that myspace's pageviews were over 50 million whereas for youtube they were only 12.6 million. This goes to show Alexa's unreliability
PageRank is also not that good for me. It only works for the google search engine, i think, and other search engines remain unaffected. What ever the case, i do not personally like both of these options.
