I have looked at the Thermomix on a number of occasions over the past couple of years trying to assess its place in a chocolate kitchen. While it can be used to make ganaches and other fillings if you take a close look at all the videos I don't see anything that involves very dense fat-heavy applications like nut pastes - marzipan, pralinés, etc.
They're not cheap either and not available directly in the US; you have to buy them from Canada but they do honor the warranty from what I hear.
If you're going to be working with nuts, and you only plan on getting one machine, the Robot Coupe is the better choice. You're going to want one with a "Blixer" designation - blender/mixer.
I make all my ganaches in the thermomix. About 4 minutes to heat, and about 20 seconds to blend- Bob's your uncle!
I do know that one of the folks I know who owns one uses it for small batch marzipan and is very happy with it.
The container of the Thermomix is 2 quart (the same as a small Blixer); however the motor is not really designed to handle dense high-fat products like marzipan and pralinés. Some things can be done, in small batches - much smaller than the max capacity of the container. Questions are - a) what kind of strain does it put on the motor (reducing working life) and b) do the quantities that can be produced match the quantity needed?
Ganaches are not hard to make over the stove with an immersion blender to guarantee the emulsion.
If I could afford to have only one it would be the Blixer. If cost were not an issue, I might go with both and experiment with producing a variety of other items such as pates de fruits.