It varies per computer and graphics card, unfortunately; so it's very hard to tell which backend is going to be faster for which computer. OGL is going to be more accurate, but in a game like Brawl, the only thing D3D9/11 screws up is the challenge menu, which shouldn't get in your way. I have to recommend using whichever backend is fastest for you in this game.
For instance, on my Core i5 3570K, GTX 760, OGL is fastest, as well as my Core i7 2760QM, GTX 560m. But on my Radeon 5850 in the first computer, D3D9 is fastest by far.
For instance, on my Core i5 3570K, GTX 760, OGL is fastest, as well as my Core i7 2760QM, GTX 560m. But on my Radeon 5850 in the first computer, D3D9 is fastest by far.