I've only briefly played white water and it didn't grab me. But I have no idea about the rule set. STTNG seems to have a lot going on and the sound is great. Mind you I never really got into this machine but my choice between the two would be star trek. Maybe try Monster Bash (but pricey)

Finally someone else who can stand-up and speak the truth. Whitewater is dishwater, stuff-all going on that makes any sense, and a pointless ramp obscuring the left flipper. STTNG looks great, cool sounds, but same it just doesnt grab me. I am a huge star trek fan, but I cant get into any flow with the pin.

Like sum1 else said, play heaps, and buy what you enjoyed. I would be buying an AFM to round out your CV and TZ.