Definitely the i3 6100 + GTX 750 Ti. Although a dual core, it'll result in much better performance with the 750 Ti against the i5 + GT 610... The GT 610 is an extremely weak GPU to today's standards and will struggle to run modern games at 1080p 60 FPS even at lower settings. The i3 6100 and 750 Ti should handle GTA V and Fifa pretty smoothly at around Medium settings 60 FPS. GTA V in particular is a very well optimised game. Hope this helps!
See here for some rather impressive performance in GTA V (High settings) on the i3 6100 + 750 Ti > https://www.youtube.com/watch?v=_SK3SiUHToQ.
See here for basically unplayable performance in GTA V on a similar performing i5 + GT 610 >
https://www.youtube.com/watch?v=0o_S9N69FN8.