Personally I would go for the Canon 100-400L, it is .5kg lighter than the Sigma 150-500, has better image quality, a quieter auto focus system and people I know that own them love them, it is also one stop faster than the Sigma at maximum zoom. The push pull system if for zoom and not focus BTW. On the downside it is an old lens design that uses an old IS system so the IS won't be as good as the Sigma's.
If you can make do with 300mm then another choice would be the canon 70-300L IS which will be heaps better than the Canon you are currently using, the Tamron 70-300/4-5.6 Di VC USD is also a lot cheaper but would certainly be a step up from the Canon lens you currently use.
My advice would be to make sure you really need to go above 300mm then either go to a shop with both and try them or hire each lens and see which one you prefer.
This image was taken this morning on the Tamron at 300mm at F8 (It has had some PP to darken the background):-
