The MT-24EX is far better than the MR-14EX. IMHO images from the MR-14EX come out a bit flat.
- You can rotate the heads to change the lighting. This is hugely important for insects that are reflective. It is also important for adjusting around obstacles near the critter.
- The heads also angle - allowing you to easily move from the 100/2.8 macro to the MP-E 65
- The heads may be placed off the front of the lens using macro brackets. This is very useful for lenses such as 180mm macros and for highly reflective things like ice/water drops where you need to dramatically change the angle of the light.
- You can control the power differential between the heads - providing more depth in the shot
One thing you need to watch out for with the MT-24EX though is the harshness of the light. I use Sto-fen diffusers to reduce this. There is also a little known hood for the MP-E 65 that helps here.
Great Info; thanks Kiris!