Here's my thought's on chains. First is when a chain stretches to the point if removing a link its time to replace it, when replacing the chain always replace the spockets at the same time. I highly recomend DID-Xring chains and any decent sprocket, I have run SUNSTAR and AFAM with good luck. After install and first adjust keep it clean and lubed and make sure you don't adjust it to tight that will wear a chain faster than anything, if any thing leave it a tiny bit loose.
When you adjust a chain to tight you don't allow enough room in the chain for the suspension to travel so when it does it stetches the crap out of the chain so it has room to travel. Almost every chain stretch problem I have seen is due to this.
I have run DID-Xrings on my last 2 dirt bikes and after the first breakin adjust they have never stetched again, first one I actually sold the bike with it still on there after desert racing on it for 1.5 seasons, the current one has about 2 Years non race time on a YZ400F dirt bike and still has not stretched, and it has about 15 more horse and a ton more torque than a stock SHEE.