The cylinder for the 686+ is actually stronger, I don't think this is a durability issue as much as a kaboom resistance issue, but the weakest points of a modern revolver cylinder are typically the notches cut into the outside of the cylinder for advancing by the "hand" or "bolt".
In a 6 shot 686 cylinder these cuts coincide with the thinnest spot in the cylider wall, with the 7 shot 686+ they are offset of the thinnest spots and the cylinder is thus stronger. If you were really going to push the pressure limits with extremely hot loads, you would actually be better off with the 686+.
Durability with reasonable loads is a different issue and some study of the timing mechanism differences would be necessary to answer this, but I feel it is a moot point.
IMHO the differences will not be significant in your lifetime. Even with a lot of use.