If you can afford the 430EX II, get it. Canon flashes are fully automatic in operation by default. So even if you bought the big 580EX II, all you'd have to do is mount it, turn it on, and start shooting. That said, the 430EX II and 580EX II also have a host of other custom settings/options that can be configured on the flash's control panel or you may be able to configure the flash from the camera's built-in menus depending on the body (40D, 50D, 7D, etc.). The settings aren't at all confusing but, you will need to take the time to read the manual and learn why we use flash. The ability to rotate your flash and/or angle it lets you "bounce" flash and create a softer, more diffuse effect that is generally more pleasing to the eye than a forward facing direct flash like the pop-up unit built into your camera.

The cheaper 270EX is fine if all you need is a slightly more powerful fill light in a small, lightweight package. There are no settings, it only has a power-switch and a rudimentary bounce/zoom features when compared to the 430EX II. This flash might be all you want on top of a Rebel (xxxD/xxxxD) series body because of it's size but, it is very limited in it's utility compared to the 430EX II and has significantly less power. These issues will become more important to you as your skill grow and it would be a shame to buy the cheap flash now only to realize you should have purchased the more advanced model in the first place.