Claws-Mail is definitely the most functional and power-user-oriented client among the Gecko-based ones (being Thinderbird, Sylpheed, Seamonkey other venerable examples). I tend to lean towards Seamonkey, mainly due to habit, but also the fact I get 2 programs within a single package instead of installing/managing them separately (e.g. Firefox + Claws-Mail / Thunderbird)