Upgrade RAM or switch to SSD?
Open up activity monitor and check your RAM usage. If it's high pressure (check the graph), or even swapping files, you will benefit from more RAM.

Any computer will benefit heavily from installing an SSD. Expect everything to go 10x faster (not kidding). Also, it will make you not having enough RAM less of a problem, as it will speed up your swap.

So it just depends on your budget and needs.

My personal opinion if you can only buy one? Get an SSD.