I havent purchased the Sigma Sting and probably wont even though I hear nothing but great things, I think my next will be the Revolt Vengeance.
I actually really like both the Covert Revolt and Venom Shock. Both are great balls and I have found to be a great 1-2 punch....For a medium THS Id really recommend the Venom Shock....if its a bit more volume then the Covert Revolt...In my opinion, you can not go wrong with either ball....Im a long time Storm/Roto guy and can hoestly say these are two of the best balls Ive ever owned...