My twopennorce for what it's worth:
This is in answer to the OP, as I don't want to get involved in any arguments please.
I'm no expert, just a happy shooter that enjoys a day out midweek and a shoot with the grandkids at the weekend.
I use 21gram cos I like it and it's cheap.
I use fibre cos its cheaper by £4.50 per 250 and it's all my dealer sells.
I shot at Park Lodge today (lovely venue btw) 500 cartridges busting clouds. I managed to really open up those rain clouds.
I bought a box of plaswad. (25)
Alternating O/U, U/O each load with plastic and fibre, neither make me a better shot
I miss just as many with both.
As far as I can tell, with my very scientific and extremely precise method of guessing, er, testing, there is no difference other than price.
For those interested, 21gram still hits the high tower birds, fast birds, blah blah blah, just a touch more lead required.
For what it's worth I had a good day with my son and really enjoyed the sport of shooting.