AS Russian and/or PolishWatch this thread
I don't know much at all about Russian A-level, other than that it has a mandatory speaking unit, and that this A-level is more popular with (privately educated) British students, so the standard of language that's expected is slightly lower than in A-level Polish.
Polish and Russian though! That's a great combination So do you speak both of these languages at home? / Do you speak one of them as your mothertongue language and then went on to learn the other language? / Are you a Western-Ukranian / Slovak who has some familiarity with both?
However if you're not fluent in the languages/one of them already, then take it! Polish I imagine would be harder as I imagine more natives take it, thus making the grade boundaries higher, but Russian would be a great A level to have if you're not fluent in it! (I'm sure Polish would be too assuming you're not fluent, but Russian would probably have lower grade boundaries as I imagine it's less taken by just natives).