I have come across this article a while ago and I agree with it

As so often is the case, the answer to this question lies partly in the eye of the beholder. For somebody from Spain learning Portuguese doesn’t present too many difficulties and lots of learners from the Netherlands find it quite easy to master the English language. As a matter of fact, different language families as well as cultural issues lead to a different approach of learners towards different languages. Due to the similar or identical characters of Japanese and Chinese, the Japanese can easily understand written Chinese. Interestingly, grasping the Chinese pronunciation has turned out to be more of a challenge for Japanese learners. All this shows, that there are many different aspects that need to be taken into account in order to pass a reasonable judgment on the matter. As English is the nearest thing to a world language there are lots of tools and textbooks that facilitate acquisition of the language, by contrast, this is not true for minor languages spoken by a smaller group of people.

While the common consensus tends to describe Chinese and Russian to be among the most challenging languages, a survey conducted by the British government has shown that this is not necessarily true. According to their results, which are based on interviews with diplomatic staff around the world, they found that on the list of the most difficult languages Basque is the top dog, followed by Hungarian with dozens of word cases. Surprisingly, among the interviewees Chinese was not considered to be hard to learn.

Russian is the easier language to learn and it is also more useful to know since Russia is a much bigger country, has more people and is more powerful. I would definitely recommend for you to learn Russian. It is the 8th most spoken language with 160,000,000 native speakers while Polish is not on the top 10 list

Maia

The only exception, he said, was when you are learning a language that is structurally similar to your own as opposed to one that is structurally different from you own. For instance, German is set up like English is, while French is not. Because of this, people who speak English often find German easier to pick up than French. They do not have to "rewire" their brains to a new grammatical system.

I would say, though, that Russian is probably more worthwhile to know. This is simply because there are more native Russian speakers than native Polish speakers! (They are both beautiful languages, though! And, from what I understand, once you pick up one it is easier to pick up the other as they are similar in structure.)