I can tell you that the
Canon EOS 60D will give you marginally better IQ than the 50D, but you'll only really see the difference in large prints. The 60D has slightly better resolution. The main reason to switch from the 50D to the 60D would be to get HD video and the tilt/swivel screen (and maybe the electronic level!). The 60D alkso gives you wireless control of an external speedlite. Noise, dynamic range, color etc are pretty similar.
So the bottom line is that if you decide to upgrade from a 50D to a 60D, you'd do it for the added features, not so much the image quality.
So far I can't really say I'm a huge fan of the Sigma Foveon sensor cameras. I've seen large prints from them and they are great, but I've seen similar large prints from Nikon and Canon cameras which were equally great. Of course I haven't seen 4ft x 6ft prints of the same subject in the same lighting with Sigma, Nikon and Canon cameras. That would be the test (and it's not one I'm likely to do myself).
There's nothing wrong with the Foveon sensor cameras per se, just that I've yet to see their real advantage. They also lock you into Sigma (and only Sigma) lenses. Again there's nothing wrong with Sigma lenses, but it's nice to have a choice.