I have the 70 - 200mm f/2.8 IS L II and very nice it is. The primary reason it's nice is the amazing corner to corner performance on a FF sensor, it's this performance which costs & is worth the money. But you have a crop frame and so long as you are not immenently planning to move to FF the the Sigma should provide results as good (shock horror) as the more expensive Canon. The main drawback with the Sigma is that it just can't maintain high resolution across the frame, not even close. This isn't so important on crop sensors, it isn't that important if all you want to do is portraits with blurred backgrounds, but it is important if you want the full potential of the lens.
I don't think it's realistic to compare the old Sigma design with the latest one, they are very different , especially the macro DG version which was not well regarded. If memory serves (and I'm not looking it up) the Sigma and Canon f/2.8 lenses are not weather sealed the f/4 IS is, which may or may not make a difference. f/2.8 makes a difference to Canon bodies focus so it's worth having, but then so is IS. The lens which will hold its value best is the f/4 IS which seem to sell on Ebay for near their new price, the Canon f/2.8 is an old design, still good, but suffers badly at resale, fetching only half it's original cost (ebay completed listings), The Sigma is just too new to tell, personally I think it's expensive, and wonder if the price will fall some what, second hand prices are holding up, but there aren't many being sold.
I'd like to reccomend something which you haven't considered, but which falls right into your price range, and that's the MkI Canon 70 - 200mm f/2.8 IS L available used for around £1000 it's likely to retain its value and there are plenty of very well looked after ones around.
Oh and one other thing - Canon teleconverters don't work with Sigma lenses, they're too wide to fit in the base of the lens.