buck460xvr beat me to it. If the crane where its marked 686 isnt marked with an "M" the gun hasnt yet been recalled and modified. S&W will do that for free. They may even do the timing work for little money (or free). Who knows maybe youll catch someone in a good mood that day. At minimum your shipping costs are covered.
I like both models of guns mentioned. Around these parts, you cant find a 686 for $450. I have a pre lock 686+ and will never part with it.
If a guy could swing it, I'd try buying his 686 no dash and keep the Security Six. Later, if need be, you should have no problem selling etither of them.
This is a tough one, both guns are great. Its a personal preference. I assume youre talking 4" barrels in both of them. I wouldnt trade a 686 pre lock, pre MIM for a Security Six. Theres about a $200 value diffence between them IMO.