For hard materials a stone will remove material faster. Softer, especially non ferrous materials a coarse flap disk will take the lead.
That is what everyone says, but there doesn't seem to be evidence from the manufacturers to support that.
https://www.youtube.com/watch?t=194&v=svB1iq4yRCE
Skip to 2:50 to see a side by side of a flap disc, grinding disc, and sanding disc. The flap disc easily and clearly outpaces the grinding disc grinding down steel.
http://www.homedepot.com/p/Diablo-4...ype-29-Conical-Design-DCXN01F/
Both the videos there also show the same thing, the flap disc is faster on steel.
In addition, many manufacturers of flap discs actually recommend (ceramic) flap discs over grinding discs for stainless steel, titanium, and other hard alloys.
If time is worth more than money, then flap discs may work better in a lot of situations. If money is worth more than time, then the grinding discs will work fine.
So it does just come down to time vs money and which do I have more of then...With Lehigh Valley flap discs only be $2 each (and they last a long time, even under heavy use), I suppose that increase in cost ($0.80 per unit) is probably worth it then. For a small time fabricator, the flap discs will probably give me great life.
I just find it odd that there is such a difference on opinion on which one is faster. Before today (when I saw those videos), I'd always assumed grinding discs were way faster (and hence only bought grinding discs because I don't care about having a smooth/glossy finish on most of what I am welding).
https://www.youtube.com/watch?t=194&v=svB1iq4yRCE
Skip to 2:50 to see a side by side of a flap disc, grinding disc, and sanding disc. The flap disc easily and clearly outpaces the grinding disc grinding down steel.
This video isn't that equal. Look at frames 2:59 - 3:00. At the 1/2 frame 2:59:15 (1/2 way between the 2 frames) you can see each weld by itself before the grinders block the view. The centre weld is way more uniform and it looks like it sits lower too but it's hard to tell. For sure the 2 outside welds have more high bumps to grind down. They should have used one continuous 12" weld and cut it into 3 pieces each 4" long, then done the comparison. It would also be easy to use crappy grinders against one good one. Not saying they did but...