It depends on the loads you shoot. If you are shooting lighter loads a longer barrel will not gain you as much. Heavier loads will see more benefit from a longer barrel.
On a ML with a very short action like the Omega, Triumph, etc, I like a 28" barrel. The overall length is still easliy managable even in tight cover. A gun like the Knight Extreme or Elite has a longer action and a 24-26" is nice to use. I am getting ready to build a Rem 700ML with a 28" barrel. It will be a tad heavy but I like velocity and will sacrafice a little weight for speed.