for a spare ball, I personally prefer plastic. I used to throw various urethane balls, like the old pearl blue hammer and more recent balls like the slate blue hammer for spares and I have found even though they were urethane, those balls would react more than I wanted a spare ball to.
I want my spare ball to be as straight as possible; especially on a sport pattern. for example, I have missed 10 pins because the urethane ball grabbed enough to hook past it, instead of rolling straight.
that said; as a new bowler to leagues, an entry level urethane may not be a bad choice; because it could be used for a strike ball when the lane conditions dictate it.