Russian-speaking Ukrainians would compete for Russia most likely. If they couldn't make the Russian team though, they'd compete for Ukraine IMO. Just look at the Kazakh tennis team, outside of Zarina Diyas every woman representing Kazakhstan in the last decade or so is actually Russian (Shvedova, Voskoboeva, Pervak, Putintseva).
So it's not actually down to preference but the level of competition within the Russian team.
Someone like Natalya Goncharova was born in Ukraine and represented Ukraine until 2010 when she switched nationalities. Not to mention because Russia is much stronger than Ukraine in women's volleyball.
Here's one for @heywoodu Kim Staelens was born in Belgium to Dutch & Belgian parents and chose to compete for Netherlands who had a much better volleyball team than Belgium (still has even though Belgium has improved since).