4
$HTTP_SESSION_VARS or $_SESSION should be of no importance.
I personally think XOOPS is way easier to work with than PostNuke. Easier overview of code, easier to see where code is outputted, few restrictions on how to name files and functions - that sort of thing.
If you don't like it and think PostNuke is better, then I would indeed recommend that you use PostNuke - choosing a CMS is mostly about personal feel with it, but personally I am not looking towards PostNuke and probably never will.