What currency should I bring to Russia?

I will be traveling to Novosibirisk, Siberia in about a week and was wondering what type of currency to bring. The place where I'm staying requested that I pay with euros but I know the ruble is the official currency. I'm bringing a credit card to pay for most things, but would like some cash for where cards aren't accepted. Which would be most commonly accepted by stores and vendors, euros or rubles?