The 500/4 Sigma gets a very good review result here:
Don't see much difference from the Canon 500/4 IS USM II, but the Sigma have less CA.
It's only a 21 MP cam of FF though:
Another positive review, with some samples, bokeh looks good to:
Another comparison Nikon 500/4 vs Sigma 500/4. Autofocus speed seem to be competitive in the Sigma
and the Nikon-version of the Sigma have a Electromagnetic Diaphragm (which the Canon version does NOT have):
A lot of reading there, but the short summary is that he finally decided on the Sigma.