TNG by a mile if you are a better than average player (or are buying the game for yourself to play).
Whitewater for guests, casual/non-players etc...
Whitewater is a very forgiving game that I do not play often anymore as I can beat the heck out of it in a seemingly never ending game when I am no a roll.
TNG has never had that problem, but I play it weird in that I generally just try to go for warp factors and combo missions in with this goal as stuff happens. I rarely make multiball or the wizard goal my priority, they just happen on occasion.
I have had both games for over a decade... FWIW. They are not bad games by any means