If you are looking for something on hooking lanes plus spare shooting I would go with urethane.  Strong cores don't help polyester enough to be a dry lane option except in the most extreme situations.  Take the urethane up to 4000 wet if you are concerned about lack of speed

As a guy that has thrown almost everything at spares, I feel that it depends entirely on your speed/rev match up.
If you are either soft with your speed, rev dominant, 2 handed or are bowling on extremely dry conditions (old wood), then I like plastic for spares.  The plastic balls with a core seem gimmicky to me, but if you go that direction I would recommend sanding the polish off of them to help blend the reaction from wet/dry and hopefully give you enough roll to experience some carry.  I am not a fan of plastic unless it is an absolute last resort since it is difficult to carry consistently.

For people who have decent ball speed and can kill their release I recommend sticking with a weaker reactive (drilled tame) or urethane.  It saves a spot in your bag for a ball which you can use for either dry or really short lanes and this is what an increasing number of tournament level players are doing.

Just one more thought.  For a pure spare ball I agree totally with polyester, but you don't need a strong core.  My answer was based on trying to do double duty where I think you would be disappointed with polyester for hooking conditions even with the strong core.  It really does depend on your game though.