The hopper capacity and roller mill sizing/design are the big differences. There's a big difference in milling a few ounces (to a pound or so) of flour versus milling enough malt for a big beer. As is the case a lot of times, you get what you pay for--not to mention, having the right tool for the job saves a lot of time and money in the end.
I'd argue that of all the brewers who have a decent mill (and have dropped the coin for one), not one would regret their purchase. My two-roller Monster Mill has been one of my best brew-related purchases and has done more to shave my brewing costs than anything else. Buying 55 lb. sacks of base malt really brings cost/beer down.