I hope you have decided to buy a new flash in order to give you more creative options, including bouncing the flash and using it off camera, and not simply to produce a brighter direct light for blinding your victims. Taking that as a given, make sure the flash you choose has a tilt and swivel head, if you plan on using it mounted on the camera. This will allow you to bounce the flash off ceiling and walls, to create nice soft light with soft shadows.
If you want the flash to do all the work, in terms of flash exposure then you will need one that can work in ETTL mode with the Canon cameras. Obviously Canon's latest flash guns will work in that mode, as will Metz's latest Canon compatible guns as well. I'm not sure about Sigma flashes and other brands. If you're happy to control flash power manually then pretty much anything that triggers at a low voltage should be fine.
The Canon 430EX and 580EX II are very good flash guns - I have a 580EX I and a 580EX II. The 580EX is indeed expensive but offers three key benefits over the 430EX....
1. It is almost a full stop more powerful. That means it can reach 1.4X further than the 430EX, all other things being equal. It also means that you can add a diffusion panel or bounce the light and (hopefully) still have power to spare when the 430EX might be at full stretch.
2. It can act as a "master" flash and can send signals to other 580EX and also 430EX units to control them all in a multi-flash setup. If you only have a one flash setup then that feature will be as much use as a chocolate teapot, but if you plan on adding to your collection then at some point a 580EX II makes sense. Of course, you could start off with a 430EX, if it is powerful enough, and make a 580EX your second purchase, when you find you have the need to control a second flash.
3. It can be swiveled +/- 180 degrees, so can cover any corner of the room for bouncing. The 430EX only goes +180/-90 degrees, so can't bounce into the corner over one of your shoulders. For more info on bouncing and the importance of tilt/swivel, have a look here....
http://planetneil.com/tangents/flash-photography-techniques/
http://www.abetterbouncecard.com/
The 580EX II does have a strobe function, to flash multiple times during one exposure. I'm not sure whether the 430X has that feature or not, but I see that as being a niche feature and not one I'll be likely to use.
If you want an easy life and are on a budget then a 430EX makes sense, although I did read that a 430EX II is about to be released, so that might be worth waiting for. If you've got the money then the 580EX II is the natural choice, as it is the daddy of the Canon flash lineup. Metz and other brands may offer better value but I'm afraid you'll have to do your own research on those as I have no personal experience.
