Here's my micro-review, after owning The Shadow for 3 days. Yeah, it's the "honeymoon period", so this doesn't really count. I'll do a detailed writeup within a month. But my initial reaction is: "why is Congo more expensive than The Shadow?"
They do have a lot in common. Three flippers, genius layouts, great music and sound, Tim Curry's voice.
But The Shadow pulls ahead in so many areas:
Stacking modes with multiballs (I think)
Video mode (both involve dodging, but it works better in TS)
Wizard mode (Final Battle is amazing)
Mini-playfield (more skill involved)
Skill shot (ramp shot, vs. aiming with the plunger)
Ramps with diverters
Sanctum lock sequence
Saucers that stick the landing
I'll admit that Congo has a few things that Shadow doesn't: the kickback, and that very satisfying shot through the pops and around the volcano. But The Shadow is my new favorite. We'll see how I feel in a few months. I won't be selling Congo, because it was my first DMD (I got it around 2003, I think). But it should feel nervous.