As far as the SBA3 vs SBA4. The A3 has the sling mount more to the rear, so I really prefer that placement. The A4 is more ridged so you may prefer that. For me it is close, but I would probably get an A4 if I was building another.
A magpul sling will attach to the A4, not quite the same as A3 bur the S4 is slightly longer and I prefer the A4. According to the law you pistols are not supposed to be shouldered....yeah right