Usually for each Canon/Nikon brand flash you can find 3rd party alternative which is 2x-5x cheaper and provides a similar functionality. The difference is mostly in build quality, and servicing options. For example: cheaper plastic, plastic vs metal hot shoe, light indicators vs lcd screen, battery door design, etc. If it brakes you might have to ship it to China.
One real difference is wireless eTTL/iTTL in the top Canon/Nikon flashes. That's where 3rd party really lags behind. But we are talking about $500-$600 flashes here.
For the seasoned pro, there are (possibly) faster recycling times, more stable flash duration and longer battery life. If you have to worry about them, you don't need a flash advice :-)
Don't go for the cheapest options, do your research. There are lots of complaints about flash being DOA or dying in a month. Check flickr discussions for example. Go for a reliable 3rd party brand that many people use. For the same price you'll get 2-3 flashes (i.e. 2x-3x more light and flexibility).