You haven't mentioned what uses you intend to put your new lens to. There are many arguments for and against IS as a purchasing factor. Personally, I am not ready to pay extra hundreds for lens with IS vs. without, as I use a good tripod most of the time, whichseems what you would have to do at 300 mm and wide open anyway.However, Sigma lenses usually have a lower resolving power than comparable Canon models. The difference in IQ is more noticeable if you're using a FF or high-resolution body.


Not quite sure which 300 f/2.8 lens you refer to, but if it's the one I know, which is quite a serious investment, there are two reasons why I would stick with the Canon equivalent in addition to higher resolving power. Firstly, as Bryan points out, there's no guarantee your third-party lens will be compatible with a subsequent Canon body you might upgrade to. Secondly, third-party converters are compatible only with their own brand. You can't use them with your other (Canon) lenses, to the best of my knowledge.


Hope this helps.