We actually have both and that is a tough decision.

GnR is pretty unique but it is less challenging from a gameplay standpoint. It is my wife's favorite even though she rarely plays. Obviously this one has a better soundtrack than most other machines, including WH2O. The magnets add a bit to the gameplay and the creative artwork and other elements make this a better machine long-term if you have room and the ability to have more than 6 machines. We have between 8-10 as things come and go and this one has been here since June 2006. It is the only Data East machine that has lasted more than a month here.

GnR appeals more to guests and casual players at our house.

Whitewater is faster and more challenging (here since Nov 2007). I play it more often than GnR, but it is not in my top 3 overall (ACDC, STTNG, Whirlwind and others are better IMO). It drains far easier than GnR. It requires a variety of shots to get Vacation Jackpot and it does flow better than GnR.

Ultimately, I suggest that you proceed with a sell and buy (unlikely trade I think), assuming that it is a for really nice WH2O. I believe that it is much more difficult to find a GnR in good shape.