Both are 6GB, 384-Bit, GDDR5 but the AMD 280X has 2048 Stream Processors and the GTX 780 has 2304 CUDA cores. Big difference is 280X is $285 cheaper.
This: http://www.newegg.com/Product/Product.aspx?Item=N82E16814127786
Or this: http://www.newegg.com/Product/Product.aspx?Item=N82E16814487040
This: http://www.newegg.com/Product/Product.aspx?Item=N82E16814127786
Or this: http://www.newegg.com/Product/Product.aspx?Item=N82E16814487040