One thing to consider is that Bernina have special feet. If you want to purchase an open toe foot (for example), at your fabric shop, it won't fit on the Bernina unless you have a special attachment to attach to the foot itself. I sew on a Bernina at my job and know this to be true. The other day I was having trouble with fabric sliding and making a bubble at the end of a long seam. I don't have a walking foot at work, but do at home. I had to bring the curtain home to sew that seam in, on my home machine. Bernina is a good solid machine though.
Where would you be going for service on the Janome? Is that location local for you or a long drive? That would be a cosideration to think about too, as you'll probably bring your machine in at least once a year for routine maintenance.