Yes, as Gretz gets at, it's not a matter of just picking a seat post design, it's a matter of getting the seat post you need to meet your needs. In addition to his questions, you should let us know where your current post clamps the seat rails and what design the post is. A pic would help too; if you're maxing out available rail with a setback post now, you won't be able to use a straight post without changing seat position. If you're changing anything else about the bike that'll play in, too.

In general, though, a seat post is not what I'd consider a 'mission critical' component. Yes, depending on how much extension you've got and how you ride, one may be able to select a post material for added comfort; I find long extension Ti and carbon fiber more comfy for long rides than aluminum, but short rides on short extension posts doesn't make a difference, and any difference in any case can be muted by saddle selection.