I have both - they are entirely different lenses, both are excellent in their own way.
The EF 100mm f/2.8L Macro IS is a 'standard' macro lens - it can focus from infinity up to 1:1 magnification. That means it's useful as a 100mm prime, e.g. for portraits and other uses for a short, moderately fast telephoto lens, as well as a macro lens. The Hybrid IS is great for use as a short tele lens, and somewhat useful for handholding macro shots (although it's less effective at macro distances). It's an easy lens to use, and seems to fit very well with 'want to jump into macro'.
The MP-E 65mm f/2.8 1-5x Macro is a very specialized lens. It starts where standard macro lenses leave off, in that the least magnification is 1:1, and it goes up to 5:1. You cannot focus beyond macro distances, this lens has no other use. It's very fun, but more difficult to use. At 5x, a grain of rice will fill the field of view.
One consequence of shooting at macro distances is that your depth of field is incredibly thin, such that you usually need to stop down to get as much of the subject in focus as possible. Another consequence is that effective aperture becomes much narrower, in terms of the amount of light reaching the sensor. The formula is effective aperture = aperture + (aperture x magnification). Those apply to all macro lenses, so at 1:1 f/2.8 with either the 100mm L or the MP-E 65mm, you're getting f/5.6 light levels. But with the MP-E 65mm, at 5x f/11 for example, you've got light of f/66. That means you almost always need to add light to your scene, so in addition to the lens itself, you'll want to budget for something like the MT24-EX Twin Lite, which is really intended for use on the MP-E 65mm. You'll need a good tripod, and probably a set of macro rails as well - the MP-E 65mm only has one ring, which controls 'focus' and 'magnification' - so, you can either preset the mag and move the camera+lens back and forth to achieve focus (thus the macro rails), or you can pick a distance and focus and get whatever mag you end up with (not as desirable, since that also changes your composition).
IMO, for getting started with macro, the 100mm f/2.8L Macro IS is the better choice. If you love macro, add the MP-E 65mm and MT-24EX down the line. If you sort of like macro, you get a lens that does macro and has other uses as well. If you really want to jump in with both feet, and can get both MP-E 65mm and MT-24EX, that combination can make some stunning images - but it takes a lot of practice to get there.
Hope that helps...