AFAIK there is no Canon 50 - 500, though there is a Sigma 50 - 500 like you mentioned. I know quite a few photographers who get great results with the lens and love it for it's versatile zoom range.
The nice thing is on a crop body you'll get an effective zoom range of 80 - 800, great for wildlife photography. What body will you use it with?
The other Sigma option is the 150 - 500 which I have (at the price we got it for I couldn't say no). It's big, its heavy, and its quite slow, that aside it represents great value for money second hand and will perform great.
My pick of the littler would be the Canon EF 100-400mm L IS USM, what it loses out on in terms of reach it gains on IQ and focussing ability. Also, L series glass brings nifty features such as weather sealing as standard.
These are all relatively expensive lenses (R12,000 - R16,000 new) so it would help knowing the following:
Your budget?
The body you'll be using the lens on?
What will you use the lens for?