May I offer you a warm welcome to the CameraLabs forums.
PhotoZone have reviewed the Sigma AF 17-70mm f/2.8-4.5 DC macro here
. I've had a very brief glance at it but I'll let you draw your own conclusions.
I own two lenses of comparable weight and when I go walking I carry camera plus the "lens for the day" in a Lowepro toploading
bag slung from my belt. The camera is very accessible and I don't notice the weight. Your choice, of course, but I think it would be a real shame to compromise the quality of your 50D's sensor unless there is no workaround you can live with.