Urethane, especially polished, can be a great spare ball option.  They are virtually indestructible and will last longer than you will. 

Plastic, or polyester, is designed to go straight, but can sometimes get brittle and crack around fingers or thumb holes.  Nothing that can't be repaired, but a consideration nonetheless.  Many cool colors to choose from.

Incidentally, both covers can "hook" if you get around them, but both can go straight as can be too.

I have used hard rubber with an old pancake core, and i currently use a Polyester.  Both are good, either find a bargain price, or find one that makes your eyes happy.  Don't expect either to last forever, and practice with them.

I just switched to an old urethane from plastic.  The plastic was very straight, but the urethane gives just enough reaction to hold/recover from a miss to the outside due to dryer boards out there - actually a little more forgiveness compared to plastic - I like it!

I believe a Mix would be a better spare ball. The pitch black is actually fairly strong from what I've seen. If you are just picking between those 2 I'd go supernatural

Any recommended UT balls?? So far from Storm will be Super Natural and Pitch Black. Which one will be a better spare ball?

Unless you're speed dominant or you face medium to medium-heavy oil regularly, neither. The SN has too much core and the Pitch Black is too dull (& too expensive).

If you have matched speed & revs or are rev dominant OR if you face lighter to dry oil patterns regularly, you'd be better off with a pancake cored polished pearl urethane like the Storm Mix.
http://www.buddiesproshop.com/p-4434-storm-mix-bowling-ball-redsilver-pearl-urethane.aspx

If you are speed dominant or usually see a good deal of oil, The Pitch Black taken to 2000 grit would work. If you're speed dominant you'd probably never have any use for a SN. Then the Mix solid or the pearl is still a better deal as a spare ball.

I say it really is up to you - how you throw and what you want the ball to do...as in not much at all for plastic or a little bit of something with urethane.  I know some folks with really high revs and/or slow speed can get good movement with plastic...

One way to think about it is "how many balls do I want to pack around?".  A ball that is only used for spares (plastic), or one I can use for spares and maybe other times (urethane).  I have tried both, and currently carry UT as it provides another option when the lanes get dry or severely broken down...and I can flatten out my release enough for 10 pins even when the lanes are dry.

There are other companies with UT balls.  Motiv has a couple - the Tank is a fairly "aggressive" one and they have the Arctic Sniper tagged as a spare ball.  Both are more $$ than a UT from any other company.  Hammer still has the Blue Hammer.  Can't recall off hand about anyone else... but if you are getting a Pitch Black, then see what it does for you and go from there.  You could get it drilled up to be weak and/or add polish.

eBay isn't a bad place to get some equip - I got my Columbia U2 from there and love that ball!

Remember when you get the ball, PRACTICE will help the most.   Learn what you need to look at, and where you need to stand, in order to cover the 10 pin.  And if you can get in the habbit/practice of covering all of your single pin spares with your plastic, takes the oil conditions out of play