Covert Tank is early to roll and doesn't seem to cover many boards from front to back. In my opinion, the core is too strong for the cover. If you take the cover to a lower(more aggressive) surface, then it just loses energy too fast. I suppose of you have higher ball speed and major rotation, then it might work okay, but otherwise, it's below average. It would probably be useful in controlling a short and heavy flat pattern(for me).
The Supra is about as opposite of a reaction as you can get. It seems to slide forever down lane, then makes a sudden turn across the lane. I find the ball to be excellent as the track on a THS drys out some and I need to move deeper. It's a great ball for bouncing off the burn and gets back with very good energy to the target. If you are looking at carry down along with a burn area, this wouldn't be a good choice. If you have a lot of oil, or even medium-heavy oil, this isn't the ball in my opinion. Even if you have the hand to make the ball read the break point, it's more than likely going to put you in over-under hell.
So, to sum it up;
Covert - Early rolling, sluggish down lane with not a lot of movement.
Supra - Slides a long way, then makes a sudden turn with good energy at the end if the conditions are favorable for what the ball is designed to do.