You can't really say one is better, they are just different. In stereo mode both sides are processed separately and in mono_l mode only the left kernels are used for processing both left and right channels. So in stereo mode the output will always be stereo, even on mono sources. If I want to keep something in mono to retain punch (for me kick and bass) I use mono_l mode. Also for the master buss it can help to keep things punchy with a dual mono setup.