People used to say that the OGL backend was more accurate than the DirectX ones, but I have no way of verifying if that's true or not. It might have applied to Dolphin when the OGL backend was better than the DirectX, but I dunno about now. DirectX has really come a long way, actually. Still, even if it is slower than the DirectX ones, it's more than fast enough.
Also, about 3.0 vs 2.0, 2.0 will probably run faster because it emulated GC/Wii games less accurately when compared to newer Dolphin revisions. 3.0 will emulate games better, with less glitches and bugs, but it'll require more from your hardware. The latest builds will give you the most up-to-date revisions of Dolphin, but still, most of the time the emulation is more accurate, hence more power needed to get it running at full-speed.
Also, about 3.0 vs 2.0, 2.0 will probably run faster because it emulated GC/Wii games less accurately when compared to newer Dolphin revisions. 3.0 will emulate games better, with less glitches and bugs, but it'll require more from your hardware. The latest builds will give you the most up-to-date revisions of Dolphin, but still, most of the time the emulation is more accurate, hence more power needed to get it running at full-speed.