OK, so first admit it, your a sick man (like the rest of us). Then come to reality and figure out what you'll do MOST. I own all Canon's USM macros, and the MP-E65.
I just sold my Markins ballhead and 85mm f/1.8, and am buying a 90mm TS-E, which I will connect with a 1.4xTC and 12mm extension tube, and do still life close-ups. I have given up trying to control depth of field with macro lenses and diffused powerful lighting.
I've got a feeling that the 90 TS-E, 1.4xTC, and Extension will give more close-up (not macro) creative power than any other combo because of DoF control. I'll let you know in 3 months, as I figure it will take me that long to learn how to use it, primarily in Tilt.
I can offer you two MP-E65 things from my experience:
1. It's hard to focus on an APS-C sensor cameras because the darker small viewfinder. Works much better on a 1-D Series or FF cameras. I use it with 1-DMkII
2. In some ways MP-E65 is more useful for abstract creative graphic arts photography than macro as I know and do macro. Creatively photographing bubbles in liquids, abstract surfaces, prismatic reflections, etc.
If a person owns no macro lens, the 100 Canon USM is the one to get first.
The 60 is stellar for pocket carrying in field with no tripod or doubling for indoor portraits,
and the 180 is for increased working distance, absolutely blurring the background, using above f/22 when needed, from a tripod.
As recommended by POTN's stellar macro man, LordV, I took the tripod collar OFF the MP-E65, bought the $27 part to move it to the Canon 100mm macro. When I use the MP-E65 on the rails, the collar sets its position back too far. It's not heavy, and works better with the camera body on the rail, not the lens anyway. So, the collar on my 100 macro either cost me $27, or $727 depending on how one views this situation.
Does all this crap make sense?
Jack