I've owned both (currently shooting 580). The 430 will do quite well if you're on a tight budget. Both speedlites offer flash coverage from 24mm to 105mm. Both also have diffusers to cover 14mm focal lengths. The big difference is the 580 offers significantly more power and can act as a wireless commander to other EX series flashes. Either will be fine with your 28-75mm lens but, I don't know if the Tamy lens reports lens focal length back to the camera to adjust the flashs' zoom heads.

The 10-20mm is a slightly different story. On your 40D, the 10-20mm lens offers the same field of view as a 16-32mm lens on a full-frame camera. What this means is you'll probably have to use the diffusers on these speedlites to orovide adequate coverage with focal lengths shorter than 15mm on your 40D. Otherwise, you should be fine.

Canon Speedlite alternatives would include units made by Sigma and Metz. The Sigma EF530 DG Super Flash falls between the Canon 430EX II and 580EX II in terms of both price and power. Sigma's EF530 DG ST Flash is pretty close in both price and performance to the 430EX II so I am not convinced of its value as an alternative. Metz offerings include the Mecablitz 58 AF-1, 48 AF-1, and 36 AF-4 shoe mount units. And if you really need a lot of power, the Metz Mecablitz 76 MZ-5 digital handle or bracket-mount flash may be something you should consider.

I personally have chosen to stick with Canon's own flashes to avoid the possibility a new camera body might not work with it consistently or that excessive voltage from the flash unit might burn out a camera. I know several professional photographers using the Sigma EF530 DG Super and all of them are quite happy. That said, I am quite happy with my 580s and I liked the 430 as well. All of these units (those from Sigma and Metz included) are E-TTL II compatible and offer varying degrees of control in terms of power, duration and direction of the flash.