I would say definitely Russian because this language is much bigger than Czech. Moreover many Czechs speak Russian=) but if you are going to work directly with Czech Republic, than Czech language will be more helpful.

Přihřeji si polívčičku. :-). Doporučuji Ti češtinu. Čeština je o špetku bohatější i složitější než ruština, takže pokud zvládneš češtinu naučíš se následně ruštinu o polovinu rychleji. Navíc náš již český jazyk zakódovaný v genech.

I must say russian

If you have to choose, between Russian and Czech, you should choise the 3 option - Polish. It is much popular than Czech, Poland is a bigger country. There's a lot of Polish speakers in the world (in the US, couple of millions, mostly in Chicago and New York), Poland, just like Czech Republic is in EU. Polish is similar to both, Czech and Russian(to Ukrainian, to be precise), so you could learn them both. But it is your private choice. Russian is an international language, very popular in the world, but not so popular in Central Europe. Very few Poles or Czechs, nowadays, speak Russian. There's a lot of Russian-speakers in Poland and Czech Republic, now, mostly Ukrainians, Belarussians (in Poland)and Russians (in Czech Republic).

It depends on your aims.