I've used several models of GR sights (Scattergun Technologies' Trak-Lock, Mossberg's GR set, the MMC sights, etc.), and find myself coming back to plain ol' rifle sights on a barrel as the most versatile all-round option. This is partly because all of my shotguns are dual-purpose: I may have a short defensive barrel on them one day, and a longer bird or clay barrel the next. If I had a GR rear sight on my receiver, I'd have to take it off every time I switched barrels, which is a no-no. On my 870's, the 20" IC RS barrel from Remington is my chosen short barrel.
I'm about to fit the XS Tritium Express sights to both my short barrels, having looked into them and other options (e.g. fiber-optic, etc.). I think I'll go for the small dot rather than the big dot, because at longer ranges, the big dot can obscure the target. At night, of course, the tritium dots are the same size in both, so there's no noticeable difference.