I don't personally use a flapper for pigeons, only an old one for crows.
I have seen others use flapper to good effect & it seems you can buy all manner of types of flapper these days.
For me the Magnet is a great all round tool and can often be the difference in making a bag or not.
A fair bit of my shooting is drawing birds off a flightline, either going out to feed or returning after a feed.
The magnet is very useful for this and helps attract the birds in my General direction. I don't believe a single flapper would have the same effect.
I have shot a fair few pigeons on laid Barley of late and found the only decoys i have needed is 2 dead birds on a magnet.
I'm sure the flapper is a great tool, but for me if I had to choose it would be the simple magnet. Not a magic tool by any means but a very useful one in many different situations.