I voted for 168 Gr.
The AAC SD has a 1:10 twist barrel, at 20". 168 will be great. Depends on what you want to do with the rifle though. For hunting, the 168 Gr Match bullets are not ideal. For target, and long range, stick with match bullets if you can.
I used t use 168 Gr Hornady's, but switched to 175 Gr Sierra Matchkings. I get slightly better accuracy, better ballistic coefficient, which matters for me in F-class and longer ranges.
Do you handload? Recommend you consider it if you do not. You can absolutely save a ton over factory match ammo, even with good components.
I use Lapua brass (expensive), Federal 210M primers (expensive), and of course 175 Matchkings (expensive, and sometimes hard to find) and I still beat the best deals on factory match.