Off the top of my head...
1. Ease of use. With the corona mill, you need to rig something up to catch the grain from the out, or make a special bucket to hold the crushed grain while you're milling it.
2. Mess. Unless you cover the end of the corona mill, chances are, you'll be finding grain parts in places you didn't think to look.
3. Gap setting. Have you seen what some people have needed to do to their corona mill in order to get a good gap set, and to hold it?
Sure, it's a cheaper mill, and can do a decent job, if you compensate for it's shortcomings... BUT, an actual grain mill (such as a Barley Crusher, Monster Mill, etc.) is designed to do a much better job, with much less work on your end, for a looooooong time...
I WAS thinking about getting a corona mill initially. After researching what I'd need to do in order to use it, and such, I opted to get the BC instead. Easy to connect my cordless drill to it, just needed a bucket (not using the bucket primary from my first hardware kit, so there we are) and I was off to the races... Easily crushes 6# in a minute (think it actually goes a bit faster than that at the default gap of .039") with the drill... Knowing myself, I knew that I'd probably get fed up with the corona mill after a couple of uses and be getting a roller grain mill. So, it would have ended up costing me ~$50 more than just getting the BC from the start. Even IF I could sell the corona mill to someone else, I'd kind of feel bad doing it... Especially knowing how it didn't perform up to my expectations.
Get what you can afford, but I see the cost difference as minor... I know there are enough people here with the corona type mill, and most will probably say it does a great job. Which I'm sure it does, for them... Just like you'll have plenty of people using roller mills that love the one they have... I have yet to see anyone post up how they regret getting a roller mill and wish they had picked up a corona mill.