The single 970 by far... not even a choice. The 280X is not in the same discussion as the 970 and having 2 does not equal double the power or double memory.
You'd be stuck with 2 cards and still only 3GB with clock speeds that don't compete with the 970. Then there is the power needed to run 2 AMD cards and the heat and noise that comes with them.
I love my Sapphire 280X but if I had the budget it would be the 970 all day long.