Abrasive grain and bonding materials they use play a lot to do with it.
Aluminium oxide is the cheaper stuff, then zirconia grade, then ceramic.
The high end ceramic gains self sharpen with use, which is pertty neat. The high end 3M discs (around 19 bucks a wheel) leave the work piece completely cool after grinding, while all the heat is ejected with the swarf.
Hardness of the grain can also be a hinderance, when grinding, the disc can become clogged and lead to poor performance and skate across the material, bluing the work piece with too much heat from friction. Too soft an abrasive, and you'll forever be changing discs.
TL;DR
Stick with a known brand, and what works for you. You can pay big money for specialty abrasives, but most of the savings at that end of town are in production costs in a factory environment (Less wheel changes and faster cut = more widgets out the door at the end of the day).
For a home gamer most 'average' wheels will do the job fine for general fab.
-Mitch
'El Burro' 2012 Defender 90.
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