It appears you (the OP) decided that The Shadow is the choice over The Simpson's Pinball Party.

Before you make the purchase, do yourself a big favor and really get some time on both machines. Ataritoday made some excellent comments and suggestion on his experiences between both games, and I totally agree. You're the one that needs to make the final decision, based on your preferences, not by any of our comments, which have been essentially split.

Both are very good games that will always have a following and be in demand. Currently, the market price for a TS is overpriced compared to what it was selling for a year ago, which was around $2500 for an above average condition example (and 2-3 years ago for just over $2000). It's also not in short supply, per se, as 4,225 units were manufactured, who knows how many still exists.

TSPP is currently around $4500 for an above average condition game, possibly even a HUO example. The price has not shifted upwards much in the past 2-3 years, as there were close to 6,000 units manufactured.

This is what's great about the pinball hobby, you will eventually own both down the road, one now and the other one a year or more later. My answer to your question is clear from my preference, I would choose TSPP over TS.