I prefer 12" x 12" because I can add more pictures. I find it easier to use this size because I tend to scrapbook in a collage style. I also use 8.5" x 11" for digi scraping, mainly because of the printing restraints of my printer.
I prefer 12x12 but I do 8.5x11 or smaller when I make digital scrapbooks because it's cheaper. I don't print my digital scrapbooks at home. I use services that print the books for me. I use various charts and reports in my albums.