Everyone just has to understand the Theremini and the Theremin are two different instruments. That is the basis of the whole problem here. Everyone is trying to fit the Theremini 100% into a theremin shoe size. Can't be done. There are plusses and minuses to both of them as with any instrument. But let's stop the nonsense that you can't play melodies on the Theremini. It just takes practice like with any other instrument. I now can get just as much dynamic response out of my Theremini with some tricks I've mastered, but I can not get the same vibrato response (but then again I can do things with the Theremini that no Theremin can do). You need to weigh what your real goals are. Either instrument may be right for you. And then again, neither may be.
Do I like playing both the same? Yes, but not in the same way. Maybe the real question is do I enjoy playing them both about the same. I have to say yes there. I can do so many cool things with the Theremini but then again that feel of the theremin is so nice! Then again I have them both and I try to be objective about what both can do for me (and they can not do the same things for me). If you enjoy playing it, you made the right decision.
So many people are asking should I get one or the other. You just can't say things like "the Theremini is a toy", "you are wasting your money to buy it". Those statements are being made by people who are only taking one side of the argument into consideration. The real story is more complex. And this is my take for anyone who is undecided. I am going to just assume $300 vs. $400 is not the deal breaker.
If all you want to do is learn how to play the theremin melodically and expressively like Clara, Peter and the other masters you hear on the Internet, if that's your real goal, and you have a good ear, I would tell anyone go buy an Etherwave and then go practice your butt off for two years if you want to get really GOOD! Can't see it happening in anything less than two years with constant practice and supreme commitment.
But if you are interested in electronic music, if you are a composer, if you are more interested in the "experimental" side of the theremin, if you are a beginner and especially a beginner to music and you would like to play tunes in key relatively soon realizing it does not have exactly the same feel and that your goal may be to get a theremin later if you like the gestalt of it, then maybe a Theremini is your best choice. You certainly can have a lot of fun with a Theremini. But just like a theremin, if all you do is mindlessly wave your hands in front of it like those silly Dorit videos, you will get bored of it quickly and boredom is the death of learning how to play any instrument.
Frankly, I hardly ever play my Theremini in non-quantized mode and I always optimize the range to what I am playing. I am convinced that quantization is worth the cost of the instrument in an of itself to help better train your hand motions for the theremin (people may disagree but seems to work for me). But that only scratches the surface. It's nice to have them both as that really keeps me from getting bored. When I get tired of the constant similarity of timbre on the theremin and get frustrated in practice I'll move over the the Theremini and vice versa. And there is no question, playing the theremin for hours can take its toll on you.
So its been 100 years, and pretty much with the exception of adding CV and going to transistors (which to my ears has only made the sound worse), not much has changed with the theremin. The Theremini is finally a major evolution in the instrument. Maybe not perfect, but I think a good first step and likely its the harbinger of the future of the theremin if the theremin is to grow.
And no one really brings up the most important point. It's sparked renewed interest in the theremin. That's good news for Theremin World. I will continue to like them both, play them both and post about both (as soon as I get my Etherwave back that got zapped with winter static).
Rich