10 MHz isn't going to really do anything for you. Not sure why ASUS even does that. I do like ASUS cards and I have an ASUS GTX580. Hits 800 gpu 2100 mem with no voltage mods at all. I can get this to 900/2100 with a volt mod without much effort. It is more than what I even need right now so I just leave it at stock. Runs ALL I throw at it.
If there is a big difference in price, go for the better deal. Don't go off the clock. They are both good companies to buy from.