Both these languages are just very confusing for anyone trying to learn Russian.
Different but with many similiarities. Russian is hard enough without this extra complication, I think!

The whole point of learning Russian, is that it is such a big language, and gives you access to so many different countries - so why distract yourself with Ukrainian or Belarussian when everyone in those countries speak Russian already?

Unless you are interested in the languages for a linguistic or cultural or family reasons of course, that would be a different story.