shadow is deeper and more complex. More difficult and definitely a players machine over congo. I like the rules and layout of congo as well. But history has always told congo was in the catagory as JM and DM for most of it's life. Shadow just barely a step beyond that. Right now congo is the flavor of the month, but i'm sure it will go back to it's place in line when the novelty wears off.
It shouldn't come down to value all the time anyway when doing trades. If you like the game, and the game you have is getting old, it's a good trade for you. you are getting something you want, and the person you are trading is getting what they want. Both parties are happy, it's a good trade.
I'd be curious to see what people would really own, if every pinball machine was valued exactly the same. Where there isn't some perceived idea of "status" games, or top 100 list (which some people buy because they think it makes their collection great or something). If people bought strictly on the basis on how fun the game was for them. I bet a lot of collections around this site would be a lot different looking.