truth of rotation, will be spindle run out.
Might have to add that one to the dodgy chinglish translation thread.

By the looks of the proxxon, it's got an ER20 spindle.
Although a MT taper will give a bit more flexibility, I doubt in this size of machine you'll have enough spindle torque to handle anything that an ER20 collet couldn't handle. ER20 can handle up to 13mm cutter shanks, and I'd doubt this small a machine will make much use of any cutter bigger than 4-5mm.