I tend to like the Urethane balls better, i have enough fingers to get it to cover plenty of boards, i just find it is the perfect ball (in this case a rhino pro). To get the ball todo anything it is totally up to you, it takes all the over and under reaction out of the equation, if you throw a weak shot, you get a weak result. It's good because it punishes you when you make a bad shot, not forgiving you like the reactives do (basically it keeps you honest). Of course a person would be silly not to take advantage of the new coverstocks, so i would choose a proactive coverstock over the reactive just because of the control they have, sure you can get them to flip and snap very hard, but there isnt anything better for when you have plenty of carrydown and there is an OOB outside of 10 board (just as long as it isnt a reverse block!)
It works both ways though, there have been a few decent coverstocks recently that have been let down by a weak core and way too complex drillings. You cant have one without the other, they both play a big part.