Graphics card are more chosen over processors in gaming, a simple dual core pentium could get your work done, but not any graphics card can game at todays pc requirements
so first of all both cpu are i5 which is good not a problem, now gpu the NVIDA has better performance go with option B
dont forget to choose your answer
If more meant for gaming, I would go with the Ideapad Z400, Most games need an DVD-ROM and the Ideapad U310 doesn't have a optical drive, the ideapad z400 is cleary faster !!