I haven't compared (though I do have the 100L, the 70-200 II, and a 25mm tube). But an extension tube has no optics, so you simply lose infinity focus and a little bit of light. Any flaws in the lens are magnified slightly, but the 70-200 II has so few... I did compare the 100L with the 70-200 II + 500D close-up lens, and found no meaningful difference in real-world shots. The close-up lens delivers higher magnification - 0.6x at 200mm, vs. just 0.36x with the 25mm tube at 70mm.
As for the flash, the Canon MT-24EX is a better option than the MR-14EX - the latter is a bit flat, great for documentary images but allows less creativity.
Another option is similar to the handholding mentioned above, but no 3rd hand needed - I've used a Manfrotto 233B telescoping flash bracket with a Giottos MH1004 mini ball head on the end, attached a flash (430EX II) via an OC-E3 cord, and that does very nice for macro shots. The bracket+ballhead allows the end of the flash to be placed right at the end of the lens hood, or extended further to light the subject from the side or behind. The setup cost me ~US$140, and over half of that was for the Canon OC-E3 cord - a 3rd party option there would bring the cost under $100, and the bracket is useful for portraits as well (it's main purpose, actually).
The following was shot with that setup on a 7D:
EOS 7D, EF 100mm f/2.8L
Macro IS USM, 1/60 s, f/8, ISO 400, 430EX II on Manfrotto 233B flash bracket via OC-E3
Heck, depending on what you're shooting, you can skip the flash. If you have a static scene, try a reflector. Here's a shot done 'on the cheap' (the goal was to illustrate a minimalist approach), and I liked the way it turned out.
EOS 5D Mark II, EF 100mm f/2.8L
Macro IS USM, 6 s, f/5, ISO 100
Desipte the 6 s exposure, no tripod was used. The 'reflector' was a gold-toned plastic charger.