The 620 is a 7 shot, and the 686 is a 6 shot; unless you get the 686 Plus.
The extra $40 for the 686+ gets you negligible more weight (620 is 37.9 oz, while the 686+ is 38.0 oz) and length (620 is 9 1/2, while the 686+ is 9 5/8). The difference is that the 620 does not have a full underlug like the 686, and the 620 uses a 2-piece barrel; a stainless steel shroud over a rifled sleeve.
S/F, Chaps