Having been contact juggling about 5 years now I feel somewhat confident making some recomendations.
For starting out (if it is indeed contact juggling they want to get
into) I would suggest a lacrosse ball for sure. In addition to
being unbreakable and unscratchable they have the added advantage of
bouncing back up towards you when you inevitably drop it while starting
They also have a slight tackieness to them which is extremely helpful while starting out.
I never found an acryllic particularly useful until I started doing multiple ball manipulations (palm spins).
Until I actually read this thread and realized it was in reference to
Origins, I was wondering if someone had been referring to my contact
juggling antics at 2001 World con!