If you reload, the 12 has significant and unquestionable advantages over the 16. Because I like to shoot my guns a lot, after a very long love affair with the 16, I came to the conclusion that a light 12 makes more sense for me.

Re popularity, I recently had the occasion to look at gun preferences reported by the members of LODGH (Loyal Order of Dedicated Grouse Hunters) 30 years ago vs currently. Back in the early 80's, 9% of those who completed the survey shot the 16. The most recent survey, looking at the 3 states with the largest number of cooperators (MI/WI/PA), 12% used the 16. So it's at least holding steady, although I might have expected to see more gain. The real jump in popularity among grouse and woodcock hunters came in those who use the 28ga: from just over 1% 30 years ago to almost 16% today. The 20ga remains the most popular choice (as it was then), from just under 50% 30 years ago to just over that mark today. The 12ga has lost a lot of favor, dropping from being favored by 40% to just over 20% today.