I have recently bought the MT-24EX to go with an MPe-65 lens. I was advised that because the MPe goes up to 5:1, the subject is ectremely close the the lens. The MT-24 lets you pivot the flash heads to give more versatility with the lighting. The ring flash I had previously didn't give me this option.
I'm still a beginner with the macro world so sure someone will be along soon.....