Natural ukrainian is different from russian by pronunciation (ukranian pr-n is more soft and melodical) and lexicon. Of course, russian speakers can understand this language. But i'd like to compare ukranian also with polish.
I think to learn both of these languages at the same time is too hard, the learner can be confused by differences and similarities. And choosing really depends on needs. Don't waste time for learing russian if you need to know ukranian.
P. S. Currently I am living in Ukraine the longest part of my life, i have studied ukranian in the school, but i cannot get good ukranian although my mother tongue is russian.