Quoted:
"I'm trying to decide which stock to put on my 870.  My goal is to have a folding or collapsable stock that will still allow me to get a good, steady hold/cheek weld for shooting slugs out to 100yds with iron sights.  I've been reading about the AR stock adapters, and also seen the collapsable wire stocks, top-folders, and side-folders.  Which would meet my goals the best?"

The SST-870 would not be appropriate for your use as it was designed to give the shooter the propper cheek weld to use optics.  This makes the cheek weld too high to use the existing iron sights.  We have a front sight post in the works that will allow you to use any BUIS/detach carry handle with the SST-870, but it will not be ready until February.

You may want to look at some of the ACE side folder stocks, I think they would fulfill your requirements effectively (unless you want to wait for the SST-870 front post to be in production).