I have thrown both, and I have to say that a Vertigo would be a better choice, but still not the best choice for heavy oil. Both balls are solid reactive, both have moderatly low RG and high Differentials. The Vertigo seems to just be the stronger of the two, and can handle slightly heavier oil.
If you truly want a ball for heavy oil, as in not a "heavy league shot", then you should look into either a low to high load particle ball (the Trifecta and the new Pure Energy come to mind) or an even more aggresive reactive than the Anomoly, such as the Thing Lives or the Pure Energy.