To use with what platform(s) and caliber(s)? I own a 7.62 SD and have ran it beside someone's SDN-6 at the range. IMO and with published specs and reviews, the SD out performs the SDN6. Either is still a good choice to use with 223-300BLK-308 and anything in between. The SD had a bit more volume due to the added length resulting in a bit better performance. If you are talking about a long range bolt gun only, it will work. But there should be better choices for that.
Either would be a great choice. The SD noticeably out performs the SDN on 300. I'm pretty sure that is the reason they brought it back. It was discontinued for a bit and prices for SD inflated. The "new" SD is better than my older one with the use of Inconel and the updated 51T mount.
I'm running one on a 9" 300 and it is awesome.
I have one and run it on 308, 300blk, and 556. I'm happy with it generally, but threaded suppressors are generally more accurate for precision work, so I have an AAC Cyclone for that.