I have and use both 100mm and 180mm, along with a couple other macro lenses. My 100 is not the L... but the USM version. I rarely use the 180mm on crop cameras. I use the 100mm on both crop and FF. Golden bee
If you had a crop sensor camera, I'd definitely suggest the 100mm lens over the 180mm... But you have a FF 5DII, so either lens is an excellent option for macro shooting, but they are different in other respects and have to be handled a little differently when shooting macro.
The 100mm is a more versatile lens. It's considerably more handholdable on any format camera, is a good compromise for general purpose macro/close-up shooting, and is more likely to serve dual purpose for non-macro working distances.
I use the 180mm in the field mostly, but usually on full frame and often with either a monopod or a tripod... typically for subjects that I don't want to get too close to... things that sting or bite or are poisonous or simply are more skittish and hard to approach. I don't find the 180mm all that big or heavy... it's just harder to hold steady due to the longer focal length, particularly if used on a crop sensor camera (where I suppose you could say it's about 1.6X harder to get a steady shot). I guess it tells you something that the 180mm includes the tripod mounting ring, while with the 100mm it's an optional accessory.
The 180mm is not fast focusing, though it has USM and a focus limiter to help a bit. It's just the nature of macro lenses that they are slower focusing.... they have to move the focus group a long, long way to go from infinity to full 1:1 magnification, and most are "long throw" focus design putting the emphasis on precise focus vs speedy focus. So IMO the 180mm is less useful for non-macro shooting, due to the slower AF, than the 100/2.8 (either the L or the USM... I haven't compared the earlier non-USM Canon 100mm).
When shooting macro, because of the focal length of the 180mm, you are dealing with pretty shallow DOF, too. So expect to need to stop it down more than might be necessary with the 100mm, and to need to use slower shutter speeds and/or higher ISO to compensate for the smaller aperture. The shot below should give you some idea how shallow the DOF can be.... just a few millimeters in this example (sorry, it's originally done on film and I didn't write down the shot data, but I probably didn't use the lens wide open):
EF 180mm f3.5L lens. EOS-3 camera. Settings unrecorded (probably Ektachrome E100VS or E200 film). Slide scanned with Nikon ED4000.
For the above, the camera and lens were resting directly on the ground. I don't think I used flash... It was bright sun.
For general use... and particularly for someone just starting out in macro, I'd suggest a lens around 100mm instead. It gives you enough working distance, but not too much, for most common situations. It's just an easier focal length to use, whether for macro or for non-macro work. Yet it gives you enough distance that allows working with some skittish subjects, or so that you won't cast accidental shadows on your subject.
The Canon 100L and 100 USM are both fine lenses. So are the Tamron 90mm, Tokina 100mm and Sigma 105mm. The Canon can be fitted with an optional tripod mounting ring and have focus limiters, and both have USM focus. Some of the third party lenses don't have any or all of these features. Also, the Canon are IF or Internal Focusing lenses, which means they don't grow longer when focused... This makes them larger to start with, though. Some of the third party lenses are smaller (when at the infinity focus setting), but not IF. "I'm ready for my close-up, Mr. DeMille"
EF 100mm f2.8 USM lens at f11. EOS 30D at ISO 200, 1/200 shutter speed. Handheld, 550EX flash.
For the above shot, I rested the camera on an empty 5 gallon paint bucket turned upside down. I handheld a single 550EX flash off to one side, using an off-camera shoe cord and with a couple layers of white gauze over the flash tube to reduce and diffuse the flash's output a little. This image is also a good example of how you can make the background go dark by using flash as the primary light source, rather than fill.
If you just want to experiment a little with macro, you don't have to buy a new lens... You actually could just get a set of macro extension tubes, such as the Kenko and use them with your 24-105. It's pretty close focusing already (about 1:4 or one quarter life size, I believe) and with 36mm extension it should be close to 1:1, I'd estimate. I havent used extensions with that particular zoom, but have with many other zooms and lenses. I have several macro lenses, but also always have macro extension tubes with me. They are just so handy to have and will work with practically any lens.
This shot below was done with a 70-200/2.8 IS and macro extension tube: Black & yellow garden spider in its web
EF 70-200mm f2.8L IS lens with 25mm macro extension tube. EOS-3 camera. Ektachrom E200 slide film, scanned with Nikon ED4000. Gitzo 1325 tripod, Kirk BH-1 ballhead. 550EX flash used for fill.
Unfortunately again, this was shot on film so I don't have all the shot data. I'd guess it's around 1:3 or 1:2, one third to one half life size. I was out looking for egrets, set up alongside a stream. Didn't plan on shooting macro, so didn't have a macro lens with me. But there were no egrets that day.... and this spider was practically right in front of me, it and its web beautifully rim lit in full sun. This is why I always have a set of macro extension tubes with me... practically any lens can do macro in a pinch, when you have ext tubes.
With lenses where one can be fitted, a teleconverter can also be useful for close-ups and macros. When you fit a TC, you increase the lens' magnification, but the lens' minimum focusing distance doesn't change so you're able to focus just as close as without the TC... giving more close-up ability. Or combine a TC with extension tubes to get even closer:Camouflage... Fence lizard
EF 300mm f2.8L IS lens, EF 1.4X II teleconverter, 25mm macro extension tube, at f7.1. EOS 30D at ISO 200, 1/100. Gitzo 1325 tripod, Kirk BH-1 ballhead, Wimberley Sidekick. Available light (no flash).
Adding extension tubes and/or teleconverter does slow down focus a little... or a lot if using several together. Each additional contact and the light falloff that occurs in extension are factors effecting focus speed. Note that technically speaking, the Canon teleconverters are usable with 70-200 zooms and the shortest prime is the 135/2 (actually they will work with the four TS-E lenses, too). Some third party teleconverters are usable with more Canon lenses... And you can use a short extension tube between the Canon TC and the lens to be able to fit it to more lenses, too.