What is your prefered Sprint duration and why?

I would go with shorter sprints. (I go with 2 weeks). From my experience with shorter iterations there are many advantages. Some of those are:

- there are more opportunities to learn from mistakes in 2 weeks than in longer iteration. Retrospectives for example are more frequent within a span of a project which would lead to improved efficiency in the team. (faster in the long run.)

- The team is more focused on what they are working on ( i.e. because of smaller iteration backlog log list) so there will be an increase in quality;

- Easier for everyone to know and understand what everyone else is doing. Example; its easier to understand and comprehend items to be worked by all members in 2 weeks than 4 weeks.

- People will follow the process better.