Both are 12 gauge DT extractor guns, correct? I am guessing the M21 will fetch a higher price, due to the WRA mystique and the shadows of the late Jack O'Connor. The NID looks to my eyes like an earlier series (1926-1930??) as the cocking pins look to be beveled. I recently bought a 1926 442455 sn range 12- DT, Ext. Grade 2 12 gauge 30" with pad- fits me quite well, has the same balance and feel of a similar 1909 LC Smith Grade 2- that has always been my "golden mean" as to both gun fit and performance.
My opinion, and it is just that - the earlier NID's tend to have better wood to metal fit-up and checkering- the engraving- not a huge issue for me, and the engraving doesn't kill birds. I'd take the NID over the M21- assuming the fit, barrel length and chokes are what you want in a bird gun- and use the $ that you might have to pay to get a M21 in similar configuration to the NID- for shells for off-season practice, possibly a hunt on a preserve come Fall.
I bought the NID from my friend and Master Gunsmith in Grand Rapids, MI- Brad Bachelder- on his recommendation- and I am happy with my purchase- my first NID. I happen to have a 12 gauge M21- made in 1948- left to me by an uncle in 1981, after his passing, and I like it- but I wouldn't spend the $ the dealers are asking for a similar gun (12 gauge with SST and AE) field grade today (aprox. $400-5000 range) just because it is a Model 21.
Just my 2 cents worth- you don't know me, nor I you, but I am known for my frugal habits- I'd rather have a middlin' good 12 side-by-side and spare change for shells and hunts at area bird preserves in the late Fall, than own a M21 or a Churchill or a H&H, and be bereft of the wherewithall to feed it, so to speak.
