I'm not sure I understand you. Of those two Ukraine is surely more likely to use a dirty bomb; purely because they don't have access to a traditional nuclear arsenal.

If Russia wants to use Nuclear weapons then they only need to enter the codes. That would probably be the final straw for western powers putting boots on the ground though.

Why would you use a dirty bomb in your own country?

Now Russia has control of the power plant and actual means to do it, if they decide that they want to depopulate that region a bit. Launching nukes might trigger a retaliatory strike, so I don't think that it was ever on the table.