I use my MTC sim in Ukraine and never think twice about the rates. The difference seems negligible to me. I'm sure there's a rate break for Ukraine with them. I just never think to ask or look at the MTC Russian site. I would encourage you to charge up your account though in Russia. Not sure how the payment kiosks would react if you tried to put in a Russian number while in Ukraine. Maybe it would be fine but best not to risk it I would say.

If you have time, go to an official MTC store and ask them. There's tons of price breaks they offer you given on where you'll be, how much you plan to call, what plan you have, how many texts you're going to send, etc. Most are accessible with just a text sent to a special MTC number. Report back what you find (fancy way of asking you to do for me what I should do for myself) and we'll all benefit!