Necrons are more likely to be first, they need it a whole lot more than Tau.
But I wouldn't hold my breath too much on keeping the C'Tan. They're just too expensive and powerful. Not necessarily a bad thing, but generally noone has fun when there's some big obscenely powerful thing on the table. It's like facing Teclis in Fantasy, you pretty much give up the magic phase to him. You win with him because you're a fucking munchkin, not because you have a good army, or clever tactics.
I remember an Apocalypse megabattle at my local store, from the end of Febuary, one end was held down by a Necron army, with a C'Tan, it took pretty much everything the Space Wolf player could throw at it to take it down. And once he did, the Necrons were slaughtered in short order, with the help of a fresh wave of reinforcements.