I own both games. If I had to sell one, it would be STTNG first. TSPP would probably be one of the last games I'd ever sell.
STTNG is a wonderful theme and very immersive and fun to play. I prefer STTNG theme-wise - I'm not even really much of a Simpsons fan. But when it comes to gameplay, there is no comparison: TSPP is one of the best pinball machines ever made.
STTNG is a hard game - it makes you be a better player, but there's not much strategy to it. The modes don't stack. It's pretty straightforward in terms of what to do. I have some issues with STTNG's design. I dislike that the video mode is not random so it's possible to memorize the moves and play perfectly every time -- I consider that a poor software design. I also think the inability to stack modes also to be a design deficiency in STTNG. Even Steve Ritchie admitted that the playfield needed a few design tweaks: the outlanes are notorious drain-monsters.
On the other hand, TSPP is the pinnacle of software/ruleset/playfield/game design. Every square inch of the game is packed with features. I've owned my TSPP for many years and I still have not beaten all the end-game modes. There are so many things you can do in TSPP that every time you play it, you can be doing something different. It's a hard game to wear out on.
Some people are more into themes. Some people are more into raw gameplay and great design and long-term playability. While Simpsons is a great theme, what makes the game shine is its unparalleled brilliance in software and ruleset design.
I pre-ordered WOZ mainly on the fact that the principal design people behind TSPP were on that project, and to that end, WOZ has proven to be every bit as exciting and deep a game as TSPP is. Keith Johnson really has no equal when it comes to pinball software design. TSPP's scoring is extremely well-balanced meaning you can play the game a dozen different ways and still have comparable scores. That is something you can't do with most pinball machines, STTNG included.