I've got the adjustable one and really like it for long runs. I've got the zepplin seat so as I adjust the air for my cheeks I can move the backrest to match. The GF likes having the pocket to put her ciggs etc. in for easy reach.

 

I got the adjustable one. I find adjusting it on the fly is a handy feature depending on the temp and the jacket I'm wearing. When I have a heavy leather on I need to push it back a bit to get comfortable.