If I was going to get a new medium lane ball from the Motiv line, what would be my best choice? Shock, Tribal Fire or something else? I'm a lefty with lower revs and a slower ball speed (14mph). Thanks for any suggestions!!

With your speed and revs, I would lean toward either the Venom Shock or Venom Panic.  The Sting, as great as it is, is a little strong.  Then again, your idea of medium might be different from mine. 

If you like to stay outside more than move inside, then the Ascent Apex could be a good option.  It is stronger than a lot of "weaker" balls and is very close in strength to the medium oil balls in Motiv's lineup.

What are you throwing successfully now?  That could give a lot of insight to the type of reaction that would suit you well.

If the GT-1 is your "go-to", then I would lean toward the Venom Shock as a replacement.  You might get a little more backend with the VS, but that can be mitigated with surface adjustments if necessary.

The only downside since I have never thrown the GT-1 is the difference in coverstock strengths Motiv has had since the GT-1's release.  The GT-1 and VS are very close numbers-wise, but not sure if a 50 hook rating on a GT-1 is the same as a 50 hook rating on a VS.

Do you think since I have a slower ball speed the solid cover of the Shock could be problematic for me? I mainly stick to pearls as I have had trouble in the past with solid covers rolling out or just not reacting as well for me as pearls.

In that case you can try the Panic, close to the Shock but goes farther down the lane, very clean on the fronts and has a smooth but angular shape on the backend. It is a great ball. And it is a Pearl.

Do you think since I have a slower ball speed the solid cover of the Shock could be problematic for me? I mainly stick to pearls as I have had trouble in the past with solid covers rolling out or just not reacting as well for me as pearls.

Personally, I would have thought the opposite, much like Tommy716.  Then again, I don't know how you play the shot you have. 

I think I would try the Venom Shock.  Very smooth.  A lot of people like this ball.  Likely will be my next ball.