I own both a 70-200/2.8 II and a 135/2 and both have a permanent place in my kit. When I want to travel light, I often take my 24-70/2.8 II and 135/2 as a very effective two lens combo.
For me the 135/2's advantages are size/weight, f/2 aperture and being pretty inconspicuous due to its size and color. The extra stop of light gathering ability is huge. The 135/2 is a terrific indoor sports lens that allows lower ISO's and cleaner, better quality pictures. Sure, you give up the flexibility of the zoom, but when not reach limited it does a great job.[/left]
Agreed, the 1,490 gram (3.28 lb) 70-200 f/2.8isII it is big & heavy. And fabulous. It's my most used lens.
But for travel? This is where the extraordinary 760 gram (1.68 lb) 70-200 f/4is gets a big mention. It's one of the all-time-great travel lenses with two notable advantages over the almost identical weight 750 gram (1.65 lb) 135 f/2. For a start, the obvious flexibility of a zoom plus the incredible IS performance of this lens. Don't underestimate IS power in helping deliver keepers. I had a stellar 135f/2 and sold it within a year. Maybe I've got slightly shaky hands, but my keeper rate with the non-IS 135 f/2 was disturbingly low compared to my 70-200 f/2.8isII.
A revised 135 f/2 with IS will sell it's socks off. It'll come eventually.
And those posts pretty much summarise the arguments and differences right there One lens shorter, less conspicuous and with the aperture advantage; the other with zoom flexibility, and IS which more than offsets the slower aperture if your subject is still.
I still feel tempted by the 135L because, when it's right, I think the f/2 gives something the 70-200/4 just can't match, but my head is saying the 70-200/4 is probably still the better choice as a travel lens because of its versatility. It doesn't feel right having two 70-200s in my kit though. If I get a chance to try out a 135L for a while at any point, I'll definitely give it a go.