Hey Sixty7a,
I faced exactly this choice when I got my 350D early in the year (75-300 IS versus the 70-200 f/4 L) and I went with the 70-200 and have been very happy with it. I thought I'd really miss the extra length, but never give it a second thought. I've got an olympus screw-on 1.7x teleconvertor (which does an OK but not great job), but I never bother to use it simply because the extra focal length isn't that important to me. Given the same choice again, I would pick the 70-200 again without hesitation. And apparently Canon have recently discontinued the 75-300 IS lens.
But... if you already have a 28-135 then maybe the 70-200 is too much of an overlap for you to be worth getting a new lens? The difference between 135 and 200mm isn't all that much, and in fact you could just use a teleconvertor on your 135 lens to get there. Which brings us back to the 75-300 IS lens, which although optically inferior to the 70-200, might be more useful?
But does it have to be between those two lenses though? I'm not saying there's anything wrong with either of them, but there are a few others that might be worth considering depending on what your needs are.
If you can afford it, there is the Canon 70-200 f/2.8. The IS version is really expensive, but the non-IS is a bit more affordable (but still by no means cheap). I guess you already know about these and, like me, can't justify the cost.
Sigma also make a 70-200 f/2.8 I think which I've heard is not quite as good as the Canon f/2.8, but still very nice. I think it costs around the same as the Canon 70-200 f/4 though, so at least its in the same $$$ ballpark. I'm not sure of my facts on the sigma though.. so don't take my word for it. Also check out the longer sigma lenses... the Sigma "Bigma" 50-500mm is a slowish but well-regarded lens in the price range as the 70-200 f/4. Optically it won't match the 70-200 L, but I think it isn't bad. It will also give you the focal range you need (and then some) if you don't mind lugging it around with you.
Depending on what your use will be, it might also be worth looking at a couple of the cheaper Canon prime telephotos. A lens which I've got my eye on at the moment is the Canon 135mm f/2.0 L prime (which costs a little more than the 70-200 f/4). Now I know it doesn't have the length or flexibility of either the 200 or 300 zooms.. but f/2.0!!!!!! Imagine the possibilities... who needs IS?

If I start doing more indoor sports shooting then this will be on top of my list. But I guess if you've already got a 28-135 lens then its not really extending your range, just giving you beautiful images at the 135 end. Other options might be the Canon EF 200mm f/2.8L which is about the same price as the 70-200 f/4. You lose the flexibility of the zoom, but gain an extra stop of aperture (and no doubt sharper images being a prime). If the extra length of a 300mm lens is important to you, the Canon EF 300mm f/4L IS prime lens isn't all that expensive considering it is a 300mm L lens with IS, and comes in around the same price as the 70-200 f/2.8 (non-IS). Still considerably more than the 70-200 f/4, but cheap considering the price territory you are beginning to tread into with those kind of lenses.
Have I confused you more now?
But.. bottom line I think they all do a fine job. It just depends on what job you want them to do I guess (and how deep your pockets are).
Cheers
Adrian