In general, when choosing between a straight or setback post (if your frame seat angle doesn’t already dictate which one you must use to achieve your desired position), a setback post will give you greater pedaling efficiency and more high-frequency vibration damping, while a straight post will give you more flex on big bumps, lower weight, and a more jarring ride on high-frequency small bumps.
When choosing between aluminum and carbon seatposts, our tests indicate that the carbon seatpost will offer more vibration damping on high-frequency bumpy surfaces and more flex for big bumps, while also being lighter; but they are more expensive.
All in all, if you don’t want to get pummeled on high-frequency vibrations as much, avoid zero-setback seatposts and gravitate either toward FSA’s K-Force Light SB25 carbon setback post or toward one with some suspension features built into it. If you’re going to be hitting really big bumps, get a Thudbuster. If you can’t accept the weight or the looks of the Thudbuster and still want some suspension on big bumps, go for a Ritchey WCS carbon post or a Cannondale SAVE Carbon. If you want a light, stiff post, get the Thomson Masterpiece setback.
For smooth roads, pick the Thomson Masterpiece setback seatpost. For bigger hits, like on rough cyclocross or on potholes or big pavé stones, we recommend a Ritchey carbon WCS post. For a long road ride, the FSA K-Force Light SB25 post is our pick, thanks to its excellent vibration damping — it is noticeably more comfortable than an oversized aluminum post, especially on long rides, dirt roads, or concrete or asphalt with lots of small cracks and expansion joints.