Okay, thank you both for your advice. 
I've read a lot about the Sigma being heavy and having slow AF, and I've also read that realistically it cannot be used without a tripod. Also, I've read that although its sharpness and general IQ are very good, they are not quite as good as that of the Canon's. Would it be a good idea for me to buy the Canon lens and perhaps later purchase some extension tubes if I needed the extra distance? I mean, a great deal of the photographs I will be taking are likely to be of captive praying mantids which aren't likely to make a run for it, so hopefully working distance won't affect me terribly...
Also, as I mentioned, I'm buying the Sigma Ring Flash too, which I believe comes with 55mm and 58mm adapters - I assume that this means it will be immediately compatible with the Canon lens?
I've never had a ring flash or even a flash for that matter, so I'm not sure how it all works! 
EDIT: How much would a separate lens hood be and what is a tripod mount? 