Standard or factory overclocked?

The short answer is to buy the standard card and overclock it.

As always there is a "it depends" here. It used to be that the stock cards could most of the time be bumped up to as fast and usually faster actually than the factory over clocked cards. The thing that has to be considered is that although its much less likely these days it is possible to get a stock card that just plain wont OC as far as the factory card or just not OC at all.
These days the niche thing seems to be upping voltages to get more stable or higher clocks, some of the stock cards don't support this so you would need a non reference card if this was something you wanted to do. The Asus CU cards are good for this as well as the MSI Hawk design which is basically built to Overclock using much better circuitry and components than a stock card and also better more robust power regulation .

Mactronix  

Normally it would be fine to get a standard card and overclock yourself. The issue is with the 6870, which is known to have relatively little headroom for overclocking. Better to get a factory overclocked one, so you get the guaranteed overclock, then overclock from there. Hopefully, a factory overclocked model will also have better components and higher chip binning to allow for higher overclocks than you could get with a standard version.