When you find two or three companies that match the profile, a few common sense questions will narrow your picks to one. Printing is a hugely complex field with a seemingly endless combination of equipment, skills and results. This is an article, not an encyclopedia so I'll narrow the focus to printing black and color inks on paper. This is a miniature version of a full-sized offset lithographic press that can handle a finished size of roughly 11 by 17 inches.
Different duplicators print different numbers of colors.
For quantities above 50,000, a faster press may save you some money. It may also be equipped to varnish, glue, cut and fold the piece automatically. The offset duplicators, sheet-fed and web presses are all capable of much the same quality, the real difference is one of speed. The more colors it prints and the faster is runs, the more a press costs and the more the printer must charge to run your job. The rule of thumb says you will get the best price on the smallest press that can handle your job.