I will do a full video about this very subject, at the end of the day, you pay your money and you take your chances. Personally, I still maintain that the Black Widow systems are incredible value. The fitting is pretty simple but it does hang off of a bracket so it's not as tidy as the Akra, which attaches to the bike directly.
If you are running at low rpm in a high gear and roll on throttle, there are flat spots but it's really not an issue. I don't believe you NEED a PCV but it's certainly a route you could take to get the fuel/air right (with a load of dyno time). I have a BoosterPlug which I will try and see if that helps.
To my ears, the Black Widow is louder and sounds more 'raw' but also more, tinny. That's the best way I can describe it. The Akra to me sounds superior and much deeper. I found the same on the MT07, the Akra is just such a deep tone. It's my favourite out of the two systems if we're just talking about sound.
i ve had this Akrapovic (S-H6R12-HAFT) for my 2014 CB650F.
The Akrapovic S-H6R12-HAFT is not EURO 4 compliant. The E-Mark counts only for EURO 3 Bikes like my old CB 650F (2014-16).
So thats why Akrapovic developed for the EURO 4 CB 650F (2017-2019) a new Exhaust called S-H6R13-HEGEHT.
Dont know about the law in England, but in Germany your type its not legal for the CB(R) 650R.
Akrapovic is still working on a legal -EURO 4 compliant-Version for the CB(R)650R.