I'd definitely look at Mongo first. Couch is ridiculously simple, too much so. It doesn't do locking because it versions every single record, so if you do updates very frequently you are going to have to clean your DB really often. Their queries are less flexible than Mongo's as well.

Couch is interesting in a way, but I feel like the set of problems that fit it well is far smaller than Mongo's. (and that that set is far smaller than SQL still, but that's another matter)

Mongo is also way way faster, though the API isn't as 'cute' by far. I feel like the great web UI for couch, along with their heavy javascript/json usage is the main reason it gets so much traction.