i suppose the big difference between the two would be cost,and maintenance.with the roller mill being higher usually in both categories.we grind feed with a hammermill,and ahay is spot on about how the hammermill cracks cattle feed,if that isn't a issue for you then would definately go hammermill, if it is, then the roller mill is your only option.also the cleanliness of the grain going in probably makes a difference also,roller mills don't like foreign material.yes they have scalpers and magnets but stuff can still sneak through and damage a set of rolls pretty quick