Greetings,
I purchased the 100mm macro and used it on my 30D and was very satisfied once I learned to use this very thin focal plain lens.
Only then when I got the 5D did this lens really shine. Now this is only my personal opinion, but I like this lens on my 5D better.
But, I believe the the focal plain is wider on the 30D because you are back further from the subject. I'm only speculating as why.
Truly a magnificent lens. I wish I had more time to spend with it....
-keith