I wouldn't upgrade from 750 Ti to 960. I myself had (have) the 750 Ti and was considering this too. But now, I have upgraded to 970. It have three times the power of 750 Ti and so, the 960 wouldn't be much of a big difference. Its more future proof too. You are able to play 1080p high settings even Witcher 3.
GTX 750 Ti to GTX 960
Difference: 121%
http://www.hwcompare.com/18409/geforce-gtx-750-ti-vs-geforce-gtx-960/
GTX 750 Ti to GTX 970
Difference: 312%
http://www.hwcompare.com/18028/geforce-gtx-750-ti-vs-geforce-gtx-970/
Witcher 3 benchmark with 750 Ti
http://www.pc-specs.com/news/16906/the-witcher-3-benchmarks-geforce-gtx-750-ti
Witcher 2 (not 3, its 2) benchmarks with other cards compared
1680 x 1050 [No AA/16xAF]
GTX 970: 147 FPS
GTX 960: 95 FPS
GTX 750 Ti: 60 FPS
http://ht4u.net/reviews/2015/nvidia_geforce_gtx_960_msi_gtx_960_gaming_2g_im_test/index23.php