Depends. If you're used to shooting film and looking for the kind of area of view you get with a 50mm lens on a 35mm format, then forget the 50mm lenses on an APS-C sensor like the 50D and get the Sigma 30 1.4 instead.
Otherwise, the Sigma 50 1.4 is a decent lens (it always disappointed me a little), but I would not drop big money on a first prime until you're 100% sure of the focal length you want.
I agree with the Sigma 30. I got a 50mm as my first prime, and it ends up being quite long for general usage. Only found it good as a short portrait lens.
The Sigma 30 is pretty much perfect as a standard prime for your 50D. I would start there.