Personally, I am sorry to be a bit rude but if you are comparing the 100 macro with the MP-E 65, wondering which to buy, then you do not really understand macro and you should therefore stick to the 100mm. These are completely different lenses. I own and heavily use both of them and they have completely different uses. I use my 100L for small things in general - flowers, leaves, knick knacks, etc. My MP-E 65 is used for small bugs. I almost never use my MP-E 65 without an MT-24EX and CP-E4 (for longer battery life + quicker firing).
Here is a rather typical shot taken with the 100L (handheld)http://500px.com/photo/7019881
Here is a rather typical shot taken with the MP-E 65 (handheld)http://500px.com/photo/7389001
While I shoot most of my macro hand held, I do take some tripod based shots. I do this most often with flowers - which means the 100L or TS-E 90. I also use a tripod for my drop photography, but that is more for repeatability than for stabilization.
Keep in mind that if you are interested in tripod based macro, besides the obvious macro rail you'll need a very flexible tripod + head. I use a Gitzo 2541EX - which is the Explorer arm type - and an Acratech GP ball head. This allows a good deal of flexibility.
For the macro rails themselves I started with the Kirk rail - which is very similar to the Adorama one (I believe they are made by the same company). I later moved to the RRS two rails system so that I can move in both the X and Y directions. This rail is more precise and stable than the Kirk rail and I like it much better. I have heard that the Novoflex is also very good, but have not used it.
The ultimate rail in terms of precision and flexibility is the Cognisys Stackshot. I own a copy and really like it. I used it to take this stacked shot. Note that I still use my RRS rails with it for X and coarse Y adjustments.Crocus
, on Flickr
Note that a bellows is significantly different from a macro rail. You can use a bellows + certain lenses to achieve similar magnification (and greater) than the MP-E 65 but you lose the flexibility because a bellows is significantly larger and general is confined to a tripod. I have seen some people use a Nikon bellows with Canon. There are other options if you really want to go that route, but personally I find the MP-E 65 so much more useful and if I wanted higher magnification I would just attach a microscope objective to the front.