Hi Smoogle,
The main difference between both GPUs are as follows;
R9 280X VS R9 280
Effective memory clock speed: 6,000Mhz vs 5000, 20% higher memory clock
Floating-point performance: 3,482 GFLOPS vs 2,964 GFLOPS, 15% better floating point performance
Clock Speed: 1,000Mhz vs 960Mhz, 4% higher clock speed
Memory clock speed: 1,500 vs 1,250Mhz, 20% higher memory clock speed