I am one of the few that can speak for both balls,etc. The reaction is so close in these two that its hard to say which is which... The both are very early reving pieces, that can be used on either medium or some heavy conditions. There is however a slight difference in maybe length of the two balls. I think personally that the Invasion goes just a tad longer than the Theory but they both however roll off the spot the same.

 

If you are deciding to buy one or the other, then Id go with whichever you can find cheaper,etc. Some of course may disagree with what I have said. These balls are becoming a little older now, so you may can find one real cheap on ebay, etc. Good luck, John