Unless your going to take the Theremin seriously, kudos if you are she a hard one to tame, I skip it as it will become nothing more than a novelty (which to me, 400 is a bit much for a novelty). That is unless your going for a home studio with funny gizmos.
I personally with go with a one of those mini korg synths you picked.
I personally pick the number 3 but that because it seem more hands on and more possibles, and I Like that.
Thank you guys. I was on the fence between the Kaossilator and the Theremin. Kaossilator might be too easy and easy to get tired of, and the theremin might be too hard to get good at. I'm leaning towards the Korg sampling keyboard (it can do a lot of loops simultaneously).
Why would any of these things not be useful? I've got congas, a harmonica, a uke, keyboards, all kinds of effects, other things I can't remember, and I use them all! Maybe I don't record with some of these things but I think as a musician It's important to have access to all things musical. Collecting these things over the years has paid off in other ways too. My family. My daughters and my wife will join me in making up songs and play these instruments.