i would try both lenses at about 100mm and see which one can focus closer. extension tubes or even holding the lens out from the camera with a shade between the camera and lens would be preferable.
I use Kenko extension tubes stacked on my 50mm f1.8 or my 85mm f1.8. You will lose af using extension tubes. You can also "make" an extension tube at home using a pringles can.
If you want to go that route, i recommend at least f12 to keep all of the subject in focus. For light, if you dont own a ringflash, use a closed gary fong on an on camera flash.
Hope that helps
Not that the window for submissions is over, here are some macro shots I took. Not the subject I submitted, but wanted to show them anyway
This is a sample of bismuth I have
yea i've always wondered about that with the tubes with no contacts or the DIY reverse mount stuff. I assume the lens is shot wide open because there is nothing telling the lens to change to whatever aperture during the shot. is this correct or is there a way to get around it?
a friend mentioned taking the lens off stopped down, but once i reminded him that lenses focus wide open and stop down for each shot if need be...he agreed that it wouldn't work.