The 50 and 55 are referring to the tire profile, the 50 is suppost to be a lower profile, but some brands 50's are close to other brands 55's, I think pirelli is one of those brands. If yout wondering if it will fit, yes absolutley. 50's are more stable at high speed but your turn in wont be as quick. But like I said, thats IF the 55 on your current tire and the Pirelli 50 are really 5mm apart.
the 55 is better when leaned over, the steeper profile gives you a larger contact patch while cornering. But if you do LOTS of commuting, I reccomend the 50. Lots of commuting on a 55 and you will get a flat top profile, which feels wierd when going off the flat top onto the side of the tire.