Juza has a good article on comparing canon and sigma TCs together:
Juza Nature Photography
His results I would agree with totally (Barring the fullframe side since I don't have a fullframe camera) since I have both sets of TCs. The difference in the 2*TCs is something that you notice when you use them side by side in a comparison, but in the field it really is not enough of a difference to worry about
My own experiences are that 1.4TCs are very good for your money and will give you a nice focal length increase with minimal image quality loss (on top end lenses with a good shot its unnoticable at times) and only 1 stop of lightloss, which should not cripple you in most situations. I nearly always have a 1.4TC on both my 70-200mm and my two macro prime lenses as well.
The 2*TCs are a different ballgame, lightloss is more noticable and image quality takes a very noticable drop. On a zoom lens its mostly not that good an option, even on my 70-200mm f2.8 IS the image quality suffers a lot (though if you use a higher end camera body you can reduce this image quality loss - 5DM2 and 1DM2/3 are the range of body you need though). Prime lenses tend to work far better, but again your looking at hte top range of prime lenses. This is not only for image quality, but also for AF since on canon cameras the bodies will not AF if the max aperture of the lens goes below f5.6 - and since you lose 2 stops of light with a 2*TC you need f2.8 lenses otherwise you will lose AF.
Note that some sigma lenses have a max aperture smaller than 5.6, but they do not report this to the camera and thus keep AF. Further this limitation is not canon being nasty, but rather the fact that AF is totally unreliable after this point.
It can be got round by using a pro range (1D) camera where the limit is f8.
Persoanlly I would say a 1.4TC is a worth addtion to your setup - though note that canon 1.4TCs will only work with their L range of lenses and even then they will not attach to them all (typically only the >100mm ones will attach). This is a physical limitation as the front of the TC sticks out.
Sigma TCs have a far lesser stickout and so will attach to more lenses in the canon range and I belive Kenko TCs have an even smaller rise - and their Kenko Pro range are supposed to be quite good optically speaking ( I haven't used on to know myself).