The Sigma lens is not a bad lens, however it's weaknesses are rather obvious when compared to the Canon L lens.
I would be tempted to get the lens.
But then again the 20 D is quite an old body by now, and I have used the 5D with the Sigma 24-70mm f/2.8 lens, not a bad combination. With a bit of RAW-processing to get rid of the chromatic abberation of that lens, it is quite a good combination. You will enjoy the true 24mm wide angle at the 5D full frame sensor.
Neither the 20D nor the Sigma lens are "bad".. in fact they are both very good even by today's standards.
Do you find anything limiting about the camera?
Do you find anything limiting about the lens?
Me personally.... I'd stick with the combo and spend the money on some nice primes. You have a fast lens when you need the convenience of a zoom. A set of primes will give you "uncompromised" image quality and even faster aperture.
I have the 4.0 version, and it's awesome...but as soon as I can I want to invest in Canon's flagship zoom...the 70-200 2.8L. I'm positive you won't be disappointed. It's super sharp, great contrast...everything.
The focal length is also very usable on a crop camera.
I don't shoot weddings, but I do shoot an advanced amateurl level, if that's possible. I photography children as my second "job." I do get some abberation with the sigma and it just doesn't seem very sharp unless I'm doing something wrong, which is entirely possible.
I have no real problems with the 20D, I've just heard how beneficial the full frame sensor is. I look at a ton of my competitors in the area and I check the file info from a photo they took (off a blog) and of course..it's a 5D.
Also, novice question: what's the benefit of using a 50mm lens instead of a zoom?
What makes the 50mm 'better' than most zooms is that it is a prime lens. That sounds sort of redundant, but let me get there.. The fact that it is a prime lens (no zoom) makes it easier to manufacture well - the only moving part is the focus, no worrying about multiple pieces lining up just right without getting distortion, etc. They put the glass in the right spot, and leave it.
But when you start talking about Canon L glass, that's a whole other ballgame.
L glass is not a whole other ballgame.... L zooms are still inferior to L primes. Hell.. some of canon's regular consumer primes will give an L zoom a run for its money.
It is all about simplicity.. designing a group of glass lenses to zoom requires a bit of sacrifice. Some will even claim that the 70-200m f2.8 IS is slightly inferior in image quality to the 70-200 f2.8 non-IS due to the fact that the non-IS version is more simple in design ( more elements /groups).