I have the Bernina 440 QE and I love it. The 550 is supposed to be the same as the 440 but without the embroidery upgrade. They found that a lot of people were buying the 440 with the BSR but not buying the embroidery feature. I didn't get the embroidery because I didn't think I was computer savvy enough to use it. The 550 is basically the same but less expensive or so I've been told. If you have a local Bernina shop that offers classes, it really helps.
As for the difference between the 2, I don't know because I've never used the Janome. I know there are a lot who do and love it too.