You probably don't need top-of-the-line. You may, in fact, not even need Canon. I've been pretty happy with my Yongnuo yn-568ex ii. You can pick up one of these, along with a set up 622C triggers for less than a high-end Canon flash.
Typically, by the way, the types of improvements you see as you move upscale with flashes are more power (see "guide number", but take published numbers with a grain of salt), and features like ETTL and HSS, both of which might not make much sense when you're starting out, but are likely to come in handy once you figure out how to use them. Higher-end flashes also tend to have flash heads that tilt & rotate, LCD screens to keep track of settings, off-camera flash capabilities, and the ability to plug in a power pack for quicker recharges. Like Canon's high-end cameras & lenses, their top-of-the-line flashes are designed to work for a living, but you might not need that kind of duty cycle & durability. That's what you're paying for, though.