If you're just starting out, I'd go with the Ether the Moog Etherwave standard, or, if you're not sure you want to plunk down 400 dollars just yet to try a theremin, the B3 is a challenging, but reliable theremin. It's a small one, and that's why it's challenging, but I have one, and take it with me on gigs as a backup. Sometimes I evin perform with the B3.
If you think you're seriouse about it, go with the Etherwave Standard.
I have heared good things about the Kees, that has a nice tonal charactor, but there is usually a long wait time to get a new one, and I have no idea how long it takes to service if you have to ship it back.