I'm building my first aluminium bike atm (having previously ridden steel) and I'm not sure whether to go for a light alu seat post or carbon. I've heard that alu frames are harsher than steel and a carbon seat post would provide a more comforatable ride that a alu seat post.
Is it worth spending the extra on a carbon seat post?
Not really, unless you like the look (and light weight) of carbon weave. IMO, you are far better served to look into your options for a comfier saddle, wheelset and tyres -- those things are actually able to deflect in the vertical plane..
There was a thread in one of the forums not so long ago (that was about as tedious as the Battle for Stalingrad) debating this very issue.