Andrzej,
It's pretty difficult for many of us on this forum to compare these two boards at this time. The Roland EM-55 has not been released in the U.S. yet. Plus the only place it has really been seed advertised is at synthplanet. I haven't seen any user reviews for it yet. You can check with some people in the arranger forum like Goerge Kaye, who is a Roland dealer. He might have some info on the unit. The PSR-550 is a pretty decent keyboard. I honestly think it's the best one in it's class and price range. There are a lot of features packed into the unit. At one time I used my computer to translate the advertisement at synthplanet and the specs look really nice. There are WAY more sounds on the EM-55, but it's hard to say if they are better than the PSR-550 because the ad didn't list how many mb's were dedicated to the sounds. Also the EM-55 has a pitcbend stick with modulation. The 550 only has a pitchbend wheel with no modulation. Many of us are hoping the EM-55 is released soon...