After much thought, I'm buying a Canon 50/2.5 macro to use with my 300d. One "issue" often referred to with this lens is the limitation to a reproduction ratio of 1:2 - but having thought about it, while this lens was designed for full frame 35mm film, I will be using it on a dSLR with the APS-C size sensor of 22x15mm.
Consider an object with a horizontal dimension of 36mm. With the lens set for a reproduction ratio of 1:2 the size of the image at the film plane, on full-frame 35mm film (36x24mm), would be 18mm wide. The same lens on the 300d, with the same lens flange to image distance of course, will give the same sized actual image, i.e. 18mm wide. But this image will be on a 22mm wide imaging chip, so while the actual (optical) reproduction ratio remains the same at 1:2, in relation to the frame size the effective reproduction ratio becomes 22/18 or approx 1:1.22. This is quite close to the 1:1 everyone seems to want, but think they need a different lens to achieve.
There is of course no true gain in resolution for this effective gain in "magnification", but surely it is true that the best things in life are free. With the 100mm you would be getting an effective reproduction ratio of 1:0.61, a lot more than the 1:1 you thought you paid for!
Or perhaps you have a different view on this?
Incidentally, my reasons for choosing the 50mm over the 100mm:
3. Fills the gap nicely between my 17-40 and 70-200
4. Greater depth of field for a given field of view due to shorter focal length
Of course I know that the working distance is less with the 50mm, and I will have to learn to live with that.