If I were in your shoes I'd go for the 100L. You don't need the extra stop when shooting cars, actually you may even need to stop down further.
And depending on the type of "car photography" you do, you may even use the macro for detailed shots, especially of the interior.
Plus not to mention image stabilization and weather sealing.
I've had both lenses but sold the 135mm. The 135mm is the king of creamy backgroud portraits, only get it if you plan on shooting at f2 often. I kept the 100L as its a great multi purpose lens, and fairs just as well at f2.8 onwards as the 135mm (apart from AF speed, which isn't too bad if you use the focus limiter).