David,
What did you expect asking M21 or Parker on the Parker forum?
What is you intention? Is is as a shooter, collecting, appreciation, upland hunting, waterfowling, clays gun or?
If you want the strongest action then between Parker and M21 it is the M21. But if you what to shoot super hot loads buy one good autoloader. The recoil will be far less.
I owned a M21 Duck which was a fine gun. I let it go and put the received funds in another Parker. With far too many long barreled 12 and 10 gauge guns my favorite fowling piece is a Parker EH 10 Gauge in which I shot 2 7/8" low pressure loads. I have had performance with 1 & 3/8 oz Nice Shot that far exceeds my shooting ability.
So, M21 or Parker? There are some great buys on Parkers in this repressed market. One might even find a M21 at a good price.
You might however ask the same question on the M21 forum. I think Dick who runs that forum also owns Parkers.
PS: While I sold my M21 Duck I still own a M23 Heavy Duck. The M23 is a Japenese made gun but it fits me perfectly. Plus, with it I can shoot any steel shot high pressure loads without worry. That stated, I haven't shot the M23 in a couple years. I shoot and hunt with classic American SxS's.
I hope this babbling helps,
Mark