Core count is not the only thing that matters in GPU power; when AMD/NVidia improve upon their graphics cards, they improve the utilization of the hardware they are able to put into the chip. Meaning that, in this case, a CUDA core on Maxwell is not going to directly translate into the same amount of compute power that a CUDA core on Pascal would. Due to NVidia improving their design and implementation, each CUDA core of a successor generation is able to deliver more performance than those on the predecessor.
Agreed with Gam3r, go with Pascal over Maxwell. They are roughly the same gaming, but you get added perks with Pascal.