R7 260x is rebranded version of HD 7790 code name Bonaire XTX.
R9 270 is rebranded version of HD 7870 code name Tahiti LE.
R7 260X has 896 Stream Processors, 56 TAU’s and 16 ROP’s just like HD 7790. It has core clock upto 1.1 GHz, 1.97 TFlops compute performance, 2GB DDR/128-bit memory configuration with 6.5 GBPS memory speed, 115W TDP with API Support of DX 11.2, Mantle, OpenGL 4.3. Memory bandwidth is 104 GB/s
R9 270 has Stream Processors 1280, Clock Frequency up-to 925 MHz, 2.37 TFLOPS compute performance, 2GB DDR-256 bit memory configuration at 5.6 Gbps speed, TDP of 150W, API support of DX11.2 / OpenGl 4.3 / Mantle. Memory bandwidth is 179 GB/s
http://www.guru3d.com/articles_pages/amd_radeon_r9_270_review,2.html
Except for that memory bandwidth and stream processors, R7 260X offers better performance.