10.75 for sure!!! if you are going SBR, might as well have it short. honestly i think 10.75 is the ideal length. velocity is still up there, its more maneuverable than a 12" and the balance of the PWS 10.75 rifle is great.
check out these ballistic numbers from hornady. you will see the difference for their rounds in different barrel lengths. (all these length barrels will be effective). I have seen people shot with .223 from 7.5"-16" and believe me shot placement is key but they all work. the effective range talk is annoying because most people are not shooting aggressors at 400 meters, second, MOST people mount an aimpoint or eotech on their rifle, meaning they are not taking shots past 250 meters. A 10" gun with good ammo will do it all when it comes to under 200 meter shooting.
http://www.hornadyle.com/products/rifle-ammunition/223-remington/62-gr-tap-barrier
If your purpose for this rifle (which I'm assuming it is), is to have fun at the range and maybe home defense/self defense, then I would definitely do 10.75"
In terms of suppressors, you will not ruin your can with a 10.75". Most people don't shoot enough to really ruin a can. I have been trying to destroy a YHM phantom and its just not going to happen. Im out of the country right now but when I get home ill post a video of full auto fire through a 7.5" on my can. if you can afford to shoot enough to ruin a can then you can afford to buy another one.
Get an AAC 556-sd or M4-2000, use the rebate for a brakeout or brake which will help protect it.
you don't need a PWS complete rifle. keep your SBR POF. Lower if pretty much all personal.
Don't get a POF upper though.
I love the 10.75" PWS and I really think you will too. great shooter. honestly it depends on what other upper you have??? If you only have one i would do 10.75". I have 10 uppers from 7.5-16" and I find myself always using the 10.75 or the 7.5" when shooting suppressed. Un-suppressed I like 14.5".