WH20 by a mile. A two stage wizard mode, that 5x multiball opportunity, the elusive vacation jackpot. I think the Star Trek theme is a little off putting to casual pinball people who aren't Trekkies as well.
Obviously STTNG isn't a bad game, but if you aren't a Star Trek fan then I'm not sure it's a keeper. It wasn't for me, but WH20 will never leave.