The 04G-P4-3967-KR (EVGA GeForce GTX 960 4GB SSC GAMING ACX 2.0+) has a 8 pin power connector, higher power draw limit (160W compared to the later card, which has 128 watt), has 2 fan for cooling and a backplate. So, the gaming SSC ACX have better overclocking friendly design.
If you are not a very serious overclocker, it is not worth $35.
By the way, the actual name of 04G-P4-1962-KR-STCK1 is EVGA GeForce GTX 960 4GB SC GAMING. (http://www.evga.com/Products/Product.aspx?pn=04G-P4-1962-KR)
You can see what are the difference between all evga 960 on evga website.
EVGA GeForce GTX 960 4GB SSC GAMING ACX 2.0+ link -
http://www.evga.com/Products/Product.aspx?pn=04G-P4-3967-KR