Factory overclocked versus reference design. Factory overclocked cards are a bit faster, but you can (usually) just overclock the reference card and get exactly the same performance.
There are a few reasons I see for getting a factory overclocked card, and they don't apply to everyone.
1. You want the performance of overclocking without the hassle of actually doing it yourself.
2. You want a guarantee of a minimum level of overclocking, since not all cards are created equal.
3. The factory overclocked card comes with a better cooler, like the Twin Frozr design from MSI.