|As previously posted, the 50/2.5 macro will need to be closer to the subject for the same magnification. The difference is not as dramatic as you might imagine, however. One significant difference between the 50/2.5 macro and Canon's other macro lenses is that the 50/2.5 only goes to 0.5x magnification, not 1.0x as with most macro lenses. Essentially, on your 40D an object a little under 2 inches long will fill the frame. At 1.0x, you could fill the frame with an object an inch long.
So, if you need the additional magnification, a 100mm/2.8 macro USM (the non-IS version) would be great choice. The 50/2.5 should work well for you if you are OK with the lower magnification.