I went to the MAC store and tried both Ruby Woo and Russian Red on me (I'm NC15/20 if it helps)-they are very close in actual color but the biggest difference to me was the formulas and intensities-Ruby Woo was a much drier formula and because of that the color was weaker and more muted, whereas Russian Red applied much creamier on and so it was much brighter and more intense and bold.
They are both blue based reds though. The MA said he much preferred Ruby Woo, whereas I liked Russian Red much better-and after I tried them both on, even he agreed I looked much better in Russian Red
So it really depends on your skintone and also your preference-if you want a bolder red, go for Russian Red, if you want a more muted classic red, go for Ruby Woo. HTH!!