I think the biggest factor is just how they would be fished, but even that is a pretty grey area. In my book, a wet fly is a sub-surface pattern presented somehow other than dead drifted (which would be a nymph). A soft-hackle is just a wet fly with some kind of a moving collar. Those are just my definitions - I'm sure someone much wiser than I has put more thought into the subject. The only things that wouldn't really match what I'm expecting for this swap would be a true streamer (like a wooly bugger), dry fly, or something that really only works as a nymph (e.g. something heavily weighted or mimicking a natural that wouldn't move on it's own, like a cased caddis). That being said, I've caught plenty of fish swinging true nymphs and saturated dry flies after they sink. Long story short - if you'd swing it and/or it has a soft hackle collar, it's good in my book.