I have no brake on my .309 JDJ, nor do I have one on my .308 XP.
The answer to whether you want one, depends upon your recoil tolerance. I do have MNP or Arrestors on my .338 JDJ No. 2, two .375 JDJ's and .45-70, and I'm glad I do.
FWIW,in my opinion the Arrestor is much more effective than the Mag-na-porting.