we have had many discussions here on this topic, and some of them have not ended well
basically, what factory overclocks does is its sort of a guarantee that the card will and is running at above stock speed 100% stable.
This brings us to the question : Is it worth paying more for something that you can do yourself? To some it is, because they prefer to mess with the card as little as possible, and would rather have a solid unit. Others, buy the stock card and mess with it themselves. This does have some risk involved, but you save the money.
Since they are the same price, i would go for the factory overclocked version. Usually the differences are not huge (3-5fps), but i would go for the faster inna heartbeat.