Using an M.2 drive is also convenient as they attach to the motherboard and take up very little space which makes them very practical in small form factor systems and laptops or in any situation where space is limited. M.2 drives can be found ranging in price from affordable (very close to standard SATA SSDs) all the way up to high cost, high capacity NVMe versions that can set you back thousands of $$.

As for GPU vs SSD, if the main purpose of your system is gaming then GPU hands down but if gaming is secondary to every day use then SSD. I would hold off on a new GPU for the moment as AMD will soon be releasing their new Polaris based GPUs which should be hugely cheaper and faster than anything currently available. Once they are released you can expect to see price cuts across the board on older products from both AMD and Nvidia. I would personally pick up the soon to be released AMD RX 480 given it's MSRP for just $200 US. 

The only thing I would add if you can squeeze the budget a little ore is an SSD for the OS, they are fairly cheap now and will make a huge difference.