This comparison is a bit less cut and dried.
The 21 is far heavier and carries fewer shots (the same number as the FN, in fact). 2.4 lb. with 0.8 lb. magazines.
The damage is a bit hard to suss out because 2d+2 pi and 2d pi+ don't compare directly well.
The range of damage for the 17 is 4-14 with an average of 9. The 21 does 2-12, average 7; but the pi+ multiplies the damage done by 1.5; giving 3-18 with an average of 10.
That extra point, on average, doesn't make up for the loss of 12 shots in the "standard" loadout or the extra 0.8 lb. (4 lb. vs 3.2 lb.).