Being that the .308 isn't a particularly large, hard kicking caliber, and you already have a fairly heavy gun, I'd say you wouldn't NEED a muzzle brake
unless you're very recoil-sensitive (nothing wrong if you are).
Having a brake obviously would make the gun kick noticeably less, which would be a definite plus if you intend to do some high-volume shooting, it may also make it easier to see where your shots impact through the scope.
What type of gun is it? and what type of brake does it have?
Some muzzle brakes
have vents or ports only on the top and/or sides of the brake, some are called "radial brakes" and have ports all around.
If you're gonna do prone shooting stay away from radial brakes! The vents on the bottom of the brake will kick dust, dirt, sand, gravel, mud, etc. EVERYWHERE when you shoot. The effect won't be as bad on a .308 as it would be on something like a .338, but it will not be pleasant.
Brakes have disadvantages too. Namely- they're LOUD. If you don't protect your hearing, you'll lose it.
Barrel length? If the 24 inch has a radial brake, get the 26 inch. Otherwise, it's a personal choice. You may prefer a shorter, quicker-handling gun; or you may want the weight and balance of a longer barrel. The difference in velocity will be slight, accuracy is what matters.