The 620 and 4" 686+ have the same frame, lockwork, and cylinder. The 620 has a two piece barrel/shroud - which keeps the barrel under tension, enhancing accuracy, some would insist. It is also a partial lug vs the 686's full lug, a personal choice in style. They both take the same grip, too. The 686+ is 2 oz heavier - all out 'front' making it muzzle heavy - the 620 'points' better/more naturally. That extra mass does keep muzzle flip down under heavy recoil. I'm not sure if they are SS or hard chromed, but the 620 has both a silver looking hammer & trigger, too.The 620 has an MSRP of $893; the 686+ of $932. My choice is the 620.
Sadly, when I went to the local pusher's to get a 620 5/08, I saw the 627 Pro - then $80 more in the display case than the 620, which they were temporarily out of. I saw the 'extras' the 627 Pro included - 8 shots, moonclip ready, springloaded front sight, etc, and it came home with me. A best buy - even at it's MSRP of $1,059. I still want a 620... a 4" to go with my oddball 5" 686+ half lugger from a few years back. Go figure.
I buy new - there isn't a thing wrong with any of my new S&Ws now - or when I bought them. All new ones were made '01 or later - and most have 'the lock'. I have shot many, many rounds - from wimpy to blood curdling - from .38 Special to .45 Colt/ACPs. My .44 Magnums have ranged from embarassingly wimpy to SAAMI maximum wrist snappers. Modern S&W revolvers are well made - and dependable. My EDC pocket protector is generally a 642-2 with the IL. A new S&W has a lifetime warranty available via an 800# call. Buy used - or, as I call it, 'Someone else's problems...', and it's on you. New likely will have the latest endurance enhancements/improvements, too. YMMV.
Stainz