Drupal 5.x vs 6.x for new platform development

Hi!

I am an MBA student doing an internship with a healthcare services startup in Philadelphia. I am helping this team strategize and set up an open source license compliant platform for the suite of services they provide.
We are debating whether to pursue development in Drupal 6.x or 5.x ? Can somebody provide more info on thsi forum? I hear drupal 6.x is not yet in good shape, i.e the views and CCK are not fully functional yet.
But then with 5.x there is the forward compatibility issues (with themes for instance).

Some of the major implementations we are contemplating right now are
-single session sign on for an integrated instance of moodle and drupal , and a common theme for the two.
-syncing organic groups in drupal to users in moodle based on enrolled courses.
-syncing taxonomy terms to moodle courses and making them searchable based on such terms.

Any advice would help. I have worked with ZOPE and Plone years ago, but I am rather unfamiliar with Drupal. So it would help to hear from the experts !

Regards
Aruna Prabhakaran
The Wharton School, Upenn