The P35C has higher quality components. It will last longer.
It consumes 30W less (which also helps with overclocking).
It needs less cooling, which is nice if your case is not cooled very well.
It has better customer service.
It allows 3 PCI slots to be used with an 8800 GTX (the Asus has a different layout and one PCI slot is blocked by the huge video card).
And of course the lemming effect. After you see it recommended 57 times you start recommending it yourself
I bought the P35C and I'm happy with it, by the way. I really needed all 3 PCI slots, but I also like the part about consuming less.