adjustable seatpost

Right now, at this very moment, 3 threads below this one on the front page, there is a nice little post (appropriate pun....) on this very subject

lol, yeah. I just put up a nice little review on this topic. Check it out if you haven't already.

To answer your question about travel, it depends on the rider. The question to ask yourself is how low do you like the saddle for your most aggressive downhill runs.

For me, 4" is good. Before I got my Command Post (when I had to manuallly adjust the saddle via QR), I never lowered the seatpost more than 2" below my normal pedalling position. My legs are long enough to move around as needed and even get my butt all the way to the rear tire without lowering the seat much. Some people I ride with like to drop their saddle a lot lower, even as much as a whole foot below pedalling height. Those folks would really benefit from getting a 5" post instead of a 4".

So just go with what you body needs. A bigger drop isn't necessarily better, just different.