German and French are frequently required or recommended in humanities programs because a lot of important scholarship was/is written in those languages. Which language you should choose nominally depends on what scholarship you are interested in. Realistically though, unless you really commit yourself, you won't develop enough of a fluency to read dense texts in the original, so it's your personal preference.
For Russian literature, French is obviously a useful language to know because of the influence France had on Russia's history and culture. Depending on what time period you're doing though, German may be more relevant.
Slav-of-Style reacted to ExponentialDecay in Russian PhD Language Question