I have them both. What would probably push me in the direction of the 100L is the IS (though I almost never have a problem with the 135 hand-held unless the light's pretty low) and, especially important to me, the very short MFD (as you would expect with a macro lens). I like being able to get up close to fine details/small things sometimes (I don't mean macro work as such). True, you probably won't need to do so that much with cars, but when I was at a vintage car show last summer with just my 24-105 I encountered several hood ornaments which made me wish I had my 100L (I'll just have to go again this year...). Being able to get closer, by the way, may obviate any superiority the 135 L may have in terms of bokeh - the bokeh quality from both lenses is superb, and when you factor in magnification, distance ratios etc. I bet there's not much, if anything, in it (didn't someone fairly recently demonstrate here that they're essentially the same in that regard?).
The only relative disadvantage the 100L has is a disadvantage all macro lenses have - focusing, especially in low light, can be slow if the next subject is significantly farther/closer than the current one.