It all depends if you want ETTL or not. I have Sigma ring flash that is exactly the same as the MR-14EX and owned the MT-24-EX. I used the MT-24 with my MP-E65 which I sold both the lens and the flash.
The MR-14EX was really designed for medical applications like dentists taking shots of teeth but as neuronatomist mentioned because you can control or ratio each tube you can do nice work with it. It is less expensive than the MT-24 and easier to work with I found.
There are also tons of DIY projects on the net. People use an on camera flash like the 580, make aluminium foil lined cardboard tube that extends and is angled to the subject just past the end of the lens.
Here is one with my Sigma Ring flash