I also just realized, while I'm writing how they compare, I got sidetracked. What I was comparing was that they are both used to add some variety to the game. While they are very similar, as I've been saying, my initial point was THAT. We can argue until we're both blue in the face about whether or not they are similar or dissimilar, but the REASONING for using/playing is the variety.

Devour:

I'd vote walls are probably about as easy to abuse, and probably more useful. Both of them, you just have to make sure not to look at things you shouldn't normally be seeing. It's the easier to deal with, though.