Should I choose Drupal 6 or Drupal 7?
While Drupal 7 is probably the "right" thing to do, there are a few good reasons to stick with Drupal 6.

One is the status of the modules you need.  Drupal 7 versions will be less mature than their Drupal 6.  They may have better features, but may also have bugs.  Some just don't have Drupal 7 versions yet.  You need to go through your modules and make this assessment yourself.

The other is how much legacy code you wish to reuse.  Yes, you can port, but that leads me to my next point.

Most of the projects I work on are sites for clients, and are firm-fixed-cost and firm-schedule.  In my case, I have to be conservative with third-part module choices.  My clients don't care if a project is late because of a bug in a module.  In their eyes, that is my responsibility.  Yes, I can debug modules and provide patches.  But, this takes time and   maintainers don't always accept patches.

All of this weighs into the version choice decision.