ahah_helper or ctools for ahah in Drupal 6? The excellent ahah_helper module (http://drupal.org/project/ahah_helper) improves the core ahah functionality to the point where it works quite nicely. 

Ctools (http://drupal.org/project/ctools), seems to bypass the core functionality totally, offering its own brand solution.

Ctools seems to be quite a bit more complicated, needing more complexity to deal with no-js situations : http://drupal.org/node/1025448#comment-4196882

Where as ahah_helper seems much cleaner/simpler to deal with : http://drupalcode.org/project/ahah_helper.git/blob/refs/heads/6.x-2.x:/ahah_helper_demo.module

So.

Which is better?
Which is more like the implementation in 7 (so we only learn once)?
You're better of using ctools in D6. Also all the ctools ajax implementation is whats in Drupal 7 known as the Ajax Framework