Do NOT get the Athlon, it is old and outdated. The fm2+ socket line is dead. For gaming the G3258 will be MUCH better, on top of that the LGA 1150 socket has a HUGE upgrade path for the future.
If you buy the Athlon you will be stuck with an old, weak and dead CPU with no way to upgrade except to start all over.
Number of cores doesn't mean a thing, currently one Intel core equals like four AMD cores. Comparing chips by core count is idiotic at best.