I have had Model 21's in 12 ga,16 ga and 20 ga, both the 12 (which I hunted with for 7 years) and the 20 were too heavy for my taste. The reason I hunted with the 12 so long was it was my first SXS and I knew the action was strong (which it is) and could handle any load. The 20 just didn't do anything for me and I even had it custom engraved with two of my dogs
so they both ended up going down the road. I picked up the 16 ga mainly for the strength of the action for the UP flurry which was 75 birds in 2 1/2 minutes, one man shooting and you had to shoot this for part of the competition and no one I talked to liked it. It was very hard on guns and I didn't want to put my Parkers through that again so I bought the 21 16 ga. it has a balance like no other 21, the 16 is built like the Parker on a 20 ga frame, so the 16 weighs less then the 20 and feels good in your hand. If your looking at a 12 ga for hunting it is Parker period the weight of the 21 will get to you by the end of the day. For shooting sports where weight is not a factor or is a plus pick the gun that feels best to you.
Threaded Mode