Yeah, but that background magnification is one of the strengths of the 200mm end of the zoom: it can take ugly or ungainly-looking stuff in the background, and magnify it, and defocus it quite a lot, rendering backgrounds that become almost abstract. it can in common language "blow out" the background, especially on portraits with the subject fairly close to the camera, and framed tightly (as in a vertical portrait with just the head and shoulders showing), and the backdrop relatively far behind them. The 135mm lens cannot do the above to the same degree: it magnifies the size of background objects to a degree, yet usually keeps them in recognizable sharpness, much of the time (depends of the DOF equation). Vertical vs horizontal framing on the same body height will usually mean a farther camera-to-subject distance on the horizontal frame.
sorry for bouncing around in this thread, but
yesterday, i was looking through some old pics for a contest and found some lens comparison pics from last year (nikkors: 60mm 2.8, 85mm 1.8, 70-200 2.8 VRI). The 70-200 at 70mm seemed to have the best color! not by a lot, but noticeable. not a high tech comparison, though.
I bought that 70-200 VRI used, it had a focusing problem, and was beyond the 30 day return period but i negotiated the full price be applied to the newer VRII which is noticeably better.
I'm sticking with my 24 & 60 mm primes (after buying and selling corresponding zooms, ouch), i can't imagine selling it for a slightly better bokeh from a prime. Although bulky, this lense is marvelous. I can imagine having a 85 1.4 also, however.
So, my answer to the OP's initial question, "Can 70-200mm f/2.8 do the same beautiful bokeh as 135mm f/2?"
is yes, but like Derrel said, at a bit longer focal length.
Assuming the Canon 70-200 is of equal quality, I'd consider it if versatility is somewhat important versus getting the best prime. But will the difference be considerable. only direct comparison will help you.
sorry, if this post just stirred your quandary and added no value, but having gone through this process with my lenses in the past couple of years and wasting money, having seller's remorse, i just wanted to share.