I just put into service a brand new Storm Super Natural and I think I will use if for almost all spares. It is so predictable that it seems to be a "natural" (pun intended) for this usage. Anybody else use a urethane ball for a spare bell, including corner pins?

I use a Brunswick Karma Urethane. Works wonders and has just enough pop to be used as a first ball on the dry.

I used an old polished urethane ball until I had trouble on the shorter WTBA patterns - the ball hooked just enough to cost me a few missed spares.  I went to plastic and havn't had those issues since....however, I do have cracking starting around the middle finger...so yeah, not awesome but whatever.