It's really a wash. They are both capable of more, the Jeff a little, the Rigby quite a bit, but it's not needed and not recommended to stay within CIP or SAAMI pressure specs. The Rigby is capable of 416 Wby loads.
The original specs for the Jeff was a 400 or 410 at about 2150, to match the 450/400 in double rifles. That's with nice low pressures, and it'll do all you need.
The Rigby was loaded with 400 at 2400 or 410 at 2370(+/-). That too works well.
The Jeff is said to have softer recoil which is understandable assuming all things are equal as far as the gun.
David
