I have the 135L in my kit and the Tammy 70-200 at work. At 135mm f2.8, the 135L is the tiniest bit sharper. But by looking at real images, you'd never be able to tell which was which. Go to f4 and they're identical. One thing I love about the 135L is that it's more of a stealth lens. I can walk around at an event (dinner, awards ceremony, etc) shooting candids from a distance and no one really notices the lens. The Tamron looks big at a small event.
The 135L will shoot at f2 and it shines there. The Tamron will shoot 70-200 at f2.8 and shines at all those focal lengths.
If I had to choose just one, as much as I love the 135L, I'd go with the Tamron. Much more versatile and you're not giving up anything in IQ.
Thank you for your input.