Are stone or metal grinding wheels better for flour?
Though I've never ground wheat, I've used metal and ceramic mills to grind spices, rice, and nuts. As far as I know you'll usually find either ceramic or steel burrs. Steel burrs though usually stainless steel, can still corrode depending on the type of steel used. Where as you typically want a harder steel for mills, you also want something corrosive resistant, but often a compromise needs to be made between one of the other.

Its known that ceramic burrs do a better jobs grinding to a consistently small grind. However I've never had an issue with a steel burr, though I've never ground anything to the fineness of OO flour.

As far as I've heard you should not grind anything in more than 1 pass. Its my understanding that its harder to grind something that's already been ground than it is to originally grind it to that same coarseness on the first run. My coffee grinder, which is a conical burr grinder, actually voids the manufacturer warranty if you do this. I assume this is because the burr can become clogged, and can wear out the motor.

In my experience I've preferred ceramic burrs for their ease of cleaning, they don't have issues with static like steel does so the burr rarely needs to be dusted off because it doesn't accumulate build up nearly as quickly.

As far as grinding to a finer coarseness I have no reason to believe that one is better than the other, and cannot think of a reason why that would be so. I've heard that ceramic is better for this, but cannot back that with any facts.

For other reasons I prefer ceramic for just about any type of burr. As far as fineness I've found that the size and quality of the burr can effect consistency the most regardless of the material its made out of.

Addition:

I've used this grinder for over 2 years now. I don't know if it will grind to the fineness of 00 flour, but its rather cheap and good quality.