All right, now I get it better.
Hands down 24EX. Reason is the 24EX heads have push button quick releases. So, you can be holding 20D & Grip in one hand, or on tripod. You take off one of the two 24EX heads from the lens barrel clamp and hold it out front, side light, or background light aimed behind subject. You can hold it as far as 24EX cable comfortably stretches, mine about 0.5 meter, from hot shoe controller on camera body.
If you intend to do a one-handed hold, stop drinking coffee, lift weights, and do yoga to get muscles and breathing coordinated. In other words, use a tripod and cable release. One hand has the cable release, the other holds one of the 24EX heads. In studio set up, the other head, or the wireless 420 slave just goes on a mini-ball head and table top pod. Taking one of 24EX heads off works well when camera is vertical portrait position.
The 24EX is kind of expensive, limited. I constantly have to re-read the manual 2-3 times. Used as I note above, sometimes you have to cut a piece of plastic milk jug and make a diffuser for the front on head.
If it were not so darn expensive, and bulky to carry in the field, I think this Novoflex bracket, used with one 550EX and a 420EX slave offers more light power flexibility with the MP65.
http://www.bhphotovideo.com …EG&addedTroughType=search
The 24EX offers expensive macro portability, not lighting power or flexibility. With a 550 and 420 slave, you can bounce the light more.
The 14EX provides a more shadowless, mellow front lighting. It is more "clinical" light, technically excellent, but not sure I like it for critters. I made the equivalent with daylight FL bulbs here, just because I was trying to show the difference between parasite exit wound holes versus spiracles:
http://aesop.rutgers.edu …ato_Hornworm_Parasite.htm
Technically decent, but flat and uninteresting photo. This was about 0.7x