I have both and find each of them to be very fun for different reasons.
Shadow is a great players game, difficult but not unreasonably so. The fan layout provides both long and short shots. It's very fast with no pop bumpers and the rule set seems more logical and easy to understand for a game of it's depth. It's a pin with some unique features that make it stand out specifically the breakout style upper playfield, the shadow ball lock and the individual ramp diverters. The DCS sound is top notch and it's an overall great package. The translite is not and issue and I rather enjoy the art deco art package. Nice looker in my opinion.
Whitewater is more approachable and if you enjoy ramps this is your game. The rules are deep but a bit confusing if you're not familar with the pin. Having said that people love it for the flow and fun factor. Kids like the bigfoot feature and it really is eye candy for the game room and of course has the best topper of any game in my opinion. I must say my initial reaction was a good one but then I became a bit uncertain if it was a indeed a keeper. After repeated playings and a better understanding of the rules and strategy I began to see the magic of the game and enjoy it just as much as shadow.
My opinion? From a gameplay standpoint it's a wash, they're both great games. From a bang for your buck perspective it's Shadow hands down. For collectability, the nod goes to Whitewater and they're more difficult to come by so let your budget, condition and availability be your guide.
Damn, I just saw this was 2 years old. Oh well, maybe someone else will find the information useful