I have both the 50L and the 135L. Both are superb in lowlight and produces beautiful bokeh. The question you need to ask yourself is which focal length will you be using more. The 50L is a "normal" lens, flexible for many uses from landscape (Yes. You'd be surprised how wonderful it can be for landscape as it frames tighter than the 35L and 24L but still covers a lot of ground), great for candid street and travel photography (It's quite compact and discreet although heavier than other 50mm's. I usually use it "street style" shooting from the chest/hip without looking through the viewfinder), and superb for portraits producing creamy bokeh and reproduces great color fidelity. It's great for "foot zooming" technique as it falls between the wide angles 24/35mm and the 85mm. It is my most used lens.
As for the 135L, it focuses very fast, produces a distinct, beautiful bokeh that many prefer. Being f/2.0, it sees even better in lowlight when compared to others at f/2.8 such as the 40mm, 100mm, or 70-200mm. It's focal length is superb for discreet street captures and yes, portraits taken with it are simply gorgeous. Subjects will thank you too as you don't need to be right in their face taking shots. Of course, you'll need the room to move around if you're shooting inside a studio or indoors.
Currently, both the 50L and 135L are on sale with the 135L being more affordable. While there are some similarities between the two, both are completely different glass. Asking yourself what your objectives are and realizing your individual shooting style should make this decision between the two fairly easy.