They're both stellar games, but both completely different. It's a shame most people experience beat up and dark TSPP's on route - cuz you truly can't appreciate the game that way. It's a great game for home use, because you'll discover new and interesting things for years due to the depth.
Shadow's rules are a bit more straight forward, but the layout is incredible and totally unique compared to other games of its era. The controllable ramp divertors are super cool and fun, the magnet trick is sweet, and the Battlefield is probably the best upper playfield ever. It's a game that just "feels great" to play.
You can't go wrong with either...I say grab the best priced/most conveniently located one that pops up & enjoy it. If you get tired of it, sell it or trade it for the other.