Vy tančíte dobře, ale já ne. - This translation should be accepted. I sent a report.
added
Tančíte dobře, já však nikoli. Literary version of the same. :)
Nikoli and nikoliv added, use "My answer should have been accepted".
Why is "tancuješ" not appropriate here?
"tancuješ" has been included in a lot of accepted variants, but missing from the main one, i.e. "Ty tancuješ dobře, ale já ne". I've added it now.