I currently own the following primes for use on my 6D (and occasionally EOS-M): 35mm IS f/2, 85mm f/1.8, 100mm f/2.8L Macro and 135mm f/2. When funds allow, I plan to replace the 85mm 1.8 with the 85mm f/1.2 II assuming something better doesn't come along by then. I also like the 50mm focal lenght, and might consider one of the new Sigma Art lenses down the road. If I had to chose a bokeh monster from available primes it would be the 85mm f/1.2 II first and 135L second.
Your 100L vs 135L:
The advantage of the 100L over 135L is that you can get closer to your subject or move them closer to you (i.e. further from the background) which can give you additional blur which nullifies the f-stop difference. Also being a shorter focal length the 100L allows you to use it in smaller venues so it becomes much more versatile on full frame. The 9-blade circular aperture in your 100L is also better than the aperture of the 135L, which is not circular and starts showing it octagonal shape by f/2.8 and become very obvious by f/4. The 100L is also weather sealed. The 100L also has IS which is of benefit which can also help with decreasing your shutter speed if you need more light. Basically, while the 135L is a cracking lens wide open, it's not very versatile and if you already have the 100L I don't see the 135L adding much value to your camera bag.
Depends on your definition of versatile, and entirely depends on the needs of the user.
In my case, for example- the 135L produces beautiful head and head and shoulder shots, and a great portrait lens if not as good as the 85, if you have space, and is great for kids especially if running around; it acts as a nice short tele when I am traveling, great for distant landscapes, zoos; it is a great lens for indoor sports where you need the FL and the f/2; it is also great for events. I just shot an acapela concert and a show, both in piss-poor light, with fantastic results even when cropped heavily.
So, for those need the wider FL or the macro capability, the 135L is more versatile. In fact, now that I have the 135L, I am considering picking up the 100 non-L for any occasional macro work I might want to do. I tried the 100L and I didn't like the bokeh- so I wouldn't pick it as a portrait lens.
+1 I own a 100L and 135L and use the 135L quite a bit more and find it more versatile for my purposes. I love the 100L for macro work, but it's bokeh is too "busy" to my eye, I prefer the bokeh of the 135. The extra stop of light is also a huge advantage to the 135, generally lack of IS is not much of an issue for me as I almost always shoot wide open, so can keep a fast shutter speed in all but the worst light. Non-circular aperture blades also not an issue as I'm shooting at f/2 or 2.2 99% of the time.
It takes 1.4x extender with no noticeable loss of IQ to give a great 190/2.8, and takes 2x in a pinch to provide a perfectly acceptable 270/4.
As much as I love my 135L, I have not had good success with extenders on it. I've used it with both 1.4x III and 2.0x III extenders and feel the results are soft with lots of CA. The same extenders work fantastically with my 70-200 2.8 II.