Both of these flashes will work on your camera. The 430EX II is just the brand new version of the 430EX with some additional features, a faster recyle time and a better mount. I'm not certain if the 430EX II is worth its 50% higher price though it should offer better performance and convenience with your XSi. Both flashes are currently being offered with a $30 instant rebate and will give you excellent results.
BTW, when shooting with flash, point it at the ceiling for a nice diffuse lighting effect and set your camera to MANUAL so that the flash will completely control the exposure. Also run a web search for "bounce card" for a simple and cheap way to improve your flash shots even more.