It's as durable but it's not even in the same league as far as fit, finish, trigger, accuracy, or any other meaningful metric.
I have both. Actually, I have 2 Service Sixe's, 1 Speed Six, 1 Security Six, 1 686-1 and 1 681.
The Service Sixes are very well made, very durable and the fit and finish is excellent.
The 686/681's (of old- I don't but new S&W anything) are also well made and the fit and finish is excellent.
The Sixes are beefier because they are cast. People will argue they are stronger, people will argue they are weaker, people will argue they are the same. But it's all opinion. The 686/681's are slimmer because they are forged but have a full under lug while the Sixes do not. Both will serve you well.
I prefer the 686/681 trigger. Even with shimming and polishing, the Six trigger is not the same as a S&W trigger- I did not say which is better because neither is "better", they are just different. The S&W tends to be very smooth throughout the pull, the Sixes stack up before they break. You can get very proficient with either, but it's a different way of shooting.
For the price though, the Sixes cannot be beat.
I am always on the lookout for a cheap Six. The last one I bought came from a Ruger employee straight home. He died and his son sold it off. Never even test fired. No box, but perfect.