90% 100-200iso; 8% 400-800iso; 2% either under 100 or over 800.
I suspect that I'm the same. When I get home tonight, I'll have a look at the exact numbers (but this might be thrown out by night-time timelapse photos, which I normally shoot at 1600). Most of my photos tend to be deliberately planned, tripod mounted shots where I want the best possible IQ. Plus lately I've been getting into long exposure shots, and low ISOs help me obtain longer exposures. ISO 100 is where I live.
Re changing brands/cameras to get better IQ. Its never really occurred to me. I'm happy with Canon and prefer their cameras and lenses. I don't really choose cameras on IQ, but on other features. My latest purchase was theoretically a downgrade in IQ - to an APS-C Fuji X-E1. But even it produces great photos. My reasons for purchasing it included it being enjoyable to use, it has a smaller/more socially acceptable body, direct access dials, better IR performance and better AF with IR filters and ND filters. I'm one of those people for whom, once image quality reaches an acceptable level, it ceases being an important deciding factor.