If you had 4GB of RAM or less, I'd say RAM. But since you have 8GB (enough for current gaming standards), the SSD is definitely the best option.
My desktop computer has 16GB of RAM in it (i5 haswell) and I still noticed a large difference when I switched my OS to a SSD drive; even against 3rd party programs designed to use extra RAM for caching to speed up platter drive access.
I also have a laptop with 8GB in it (i5 broadwell) and it felt nearly useless due to slow loading with its 5400 rpm drive until I replaced it with a SSD as well.