Congo is worth more if both conditions of machines are completely equal.
Rarity of Congo, and inability to find parts is what raises its cost.
Gameplay quality is about the same, although I prefer "The Shadow", as I sold my Congo YEARS ago to another collector.

Congo sometimes is worth EVEN MORE in certain situations.

Unnerving and soft, the unsettling music that stands out in pinball history is BSD "Tolling of the Bells" at game over.
If collectors are not familiar go find a BSD to play, as it one of the two game over routines it showcases.
You will not forget it, even women like it, and the game itself scares young children if the player is good.

Congo does not have "fantastic" music or sound, every time Amy talked I wanted to shut the machine down.
It is marginally better than Data East's Jurassic Park with a much improved sound system.

The Shadow music in sync with game play, and the sound clips are much better.
The nod goes to The Shadow in this case.