I looked into these models, even though I only support US products, because I was curious myself. Apparently, there's a great deal of difference. I don't know which country you're in, but it appears that the B550 isn't primarily an English-speaking market's device. Otherwise, the B550 has a few more inputs and has Smart Hub for the TV function, which requires more hardware on the inside. Hope that helps!
I am looking at these two models and apart from age of release I can't tell the difference. What is the 550 model giving me over the older 300?