I remember reading some articles on brakes that were developed for the Army - declassified information. Baffles brakes tend to have the highest efficiency in the 70 to 80% range. Usually you need at least two baffles to get this efficiency, a third only adds about another 5%. The baffles need to be at least 10x caliber in area, more is better, but there is a point efficiency falls off. Ideally, you have a rolling curve to redirect the gas back, like on the WWII tanks. If I remember correctly the first baffle should be one caliber from the muzzle and the second baffle two calibers. Brakes like those for Barrett 50 cal look like this, although they are evenly spaced. Perforated brakes, the ones with all the holes in them, tend to perform in the 40-50% range of efficiency. Hole types didn't seem to make much difference, lots of small holes, or few large ones. If you go with a perferated brake, make sure it isn't too small in diameter, ie, don't reduce diameter to match barrel diameter, unless you have a bull barrel. I have seen brakes ruptured on high pressure calibers like the 300Mag when cut too thin.
I don't have a lot of experience with different manufacturers. But based on what I read, something like the JP Enterprise baffle brake would probably have high efficiency.
http://www.jprifles.com/1.4.2.php
The two baffle brake was used a lot in the past, and still is, on military cannon. I have heard claims of some people who have developed perferated brakes for the military that are as efficient, but I am not convinced its possible.
The Barret brake looks maybe a bit nicer than the JP Enterprise but I don't think the sell the brake individually.
I've always been fascinated on how well a good brake reduces recoil. But there tends to be a lot of hype, and few real performace tests comparing the various offerings.
One last thing, a brake is a gas pressure device. It has limited ability to work with low weight charges and low pressure. High volume high pressure gas makes it work best. So while a 45-70 would have some benefit, your 300 Mag would excellent. Even then, use the slower burn rate powders to get max gas volume.
Interested in what others have to say.