For short term, the GPU is a better choice. On an i3, you'll get a decent graphic increase without too much of a bottleneck. However, come next video upgrade a better CPU (i5 or better) will be required as well to not have a bottleneck.
For long term, the CPU is the better choice as you more than likely will only need to replace the GPU for the next couple of graphic generations since performance jumps have been stagnant on the CPU generation front.