The three amps you ask about are all very good, but there are some noticeable differences in character.
VTM200- these were a breakthrough for ARC. They had a combination of the best control and extension at the frequency extremes of any ARC amp until that point. They sounded more neutral than the original version of the REF300. What they do not do at all (even less than is typical for ARC) is sound tube like. Some may not like this. They almost sound more like a great solid state amplifier. Not very liquid or pretty, but very accurate and fast.
REF300mkII - The mkII was redesigned after the VTM200's broke new ground. Once this was done, the REF300 achieved and even higher level of musicality and power combined with the improved control and extension at the frequency extremes that was pioneered in the VTM200's. The REF300mkII, however, still sounds friendlier and more tube like than the VTM200's.
REF210 - This current REF series is another breakthrough for ARC for they have now officially eliminated any trace of "silvery" or "white" sonic artifacts in their high frequencies. The REF210 has a more liquid and grain-free presentation than the other amps mentioned above. The sound is much more tube-like in this way, yet paradoxically very controlled and neutral with excellent extension at the frequency extremes. Once you get used to the sound of the REF110, 210 or 610T, the older amps sound somewhat grainy, congested and analytical by comparison.
I am an ARC dealer (and a huge fan of the new VS115)
VTM200- these were a breakthrough for ARC. They had a combination of the best control and extension at the frequency extremes of any ARC amp until that point. They sounded more neutral than the original version of the REF300. What they do not do at all (even less than is typical for ARC) is sound tube like. Some may not like this. They almost sound more like a great solid state amplifier. Not very liquid or pretty, but very accurate and fast.
REF300mkII - The mkII was redesigned after the VTM200's broke new ground. Once this was done, the REF300 achieved and even higher level of musicality and power combined with the improved control and extension at the frequency extremes that was pioneered in the VTM200's. The REF300mkII, however, still sounds friendlier and more tube like than the VTM200's.
REF210 - This current REF series is another breakthrough for ARC for they have now officially eliminated any trace of "silvery" or "white" sonic artifacts in their high frequencies. The REF210 has a more liquid and grain-free presentation than the other amps mentioned above. The sound is much more tube-like in this way, yet paradoxically very controlled and neutral with excellent extension at the frequency extremes. Once you get used to the sound of the REF110, 210 or 610T, the older amps sound somewhat grainy, congested and analytical by comparison.
I am an ARC dealer (and a huge fan of the new VS115)