Poznan or Wroclaw?
Wroclaw is my favourite of the two (although last time I was there I was on a stag do, so I’ll need to wait a while before I’m allowed to return). Auschwitz isn’t too far away of course, but within Wroclaw I can recommend both the old town and Cathedral Island. None of the museums are that great I think – it’s more about soaking the place up.
I’ve not visited either properly (unless travelling through Poznan on a coach in the dead of night counts…) but I’ve heard lots of good things about Wroclaw.