If you are shooting with 55 ft/lbs or better, you can use some of the smaller diameter mechanicals, 1 1/4" or less with good results. I personaly feel though that if you can get fixed blade heads to shoot well, then in most situations you will be better of with a fixed blade head. I am not anti-mechanical, but I do have a prefernce for fixed blade heads, and I feel if you have to ask the question, a fixed blade head will most likely be a better choice.

TAKE YOUR KIDS HUNTING AND YOU WON'T BE HUNTING FOR YOUR KIDS