Hey,
The Shadow was the first DMD I owned. When I got it, it fell into the somewhat disregarded pile and people on RGP always complained about the translite. That was in June of 2011, if I recall correctly? Since that time, it's moved up in price a lot, and people have decided they like it. It is a really good game, but it is just brutally hard. I beat the wizard mode once and only once in the two and change years I had it. After that game, I shut it off, and sold it. It is a very rewarding game to beat, but I felt I'd never beat it again, and it had moved enough in price that it made sense to sell.
I'll get philosophical: The best analogy I can make is that it's like playing golf. Imagine having your best game of golf. It doesn't happen often, and it doesn't mean you've beaten the game of golf and that's it. It's more so the course has yielded to your continued effort that one particular time. You could go out the next day and play like crap.
The Shadow is the same. Me beating it wasn't me "winning" the game. It was almost like the game conceding that I had put in enough time and effort that, just this once, I was going to see it through to the end. It's not going to be for everyone, but if you're stubborn, it's such a worthwhile game.
With that said, in similar condition, I'd probably buy Congo, and here's why. The Shadow has moved up a lot in price, and to me, it didn't represent the value it once did. I personally think that in the price range, it makes more sense to buy a "value" game, or fixer-upper, than to buy a game that has run up in value to what may be its absolute peak. Where it sits now, Congo still has somewhere to move price-wise, and is likely going to be cheaper than a The Shadow anyhow.
Luke