R9 270X is better overall by 10%
Lets Compare :
AMD R9 270x vs Nvidia GTX 660
1) In terms of overall gaming performance, the graphical capabilities of the AMD Radeon R9 270X are very slightly better than the Nvidia GeForce GTX 660.
The Radeon R9 270X has a 20 MHz higher core clock speed and the same number of Texture Mapping Units as the GeForce GTX 660. This results in the Radeon R9 270X providing 1.6 GTexel/s better texturing performance.
2) The Radeon R9 270X was released over a year more recently than the GeForce GTX 660, and so the Radeon R9 270X is likely to have better driver support, meaning it will be more optimized for running the latest games.
3) The Radeon R9 270X has 35 GB/sec greater memory bandwidth than the GeForce GTX 660, which means that the memory performance of the Radeon R9 270X is noticeably better than the GeForce GTX 660.
Hope this helps.
If u can raise the budget, go for GTX 760 which is significantly better than R9 270x.