Depends on how you use your Mac, and do you feel that's slow (very OSX version dependent).
I my personal experience, anything after OSX 10.7, SSD sway better.
If you mainly use it to run some long time operation (something only require few clicks and then let it run itself for few hours. e.g. video encoding), you will benefit from RAM more than SSD.
However, if you only use it to do a lot of short time operation (you have to keep using your trackpad, click here, click there...). Then SSD should gives you much more value than the RAM.
And one good thing about upgrading SSD is that even you run out of RAM, now your SWAP is on the high speed SSD (especially good for small files read write), the performance impact will be much much lower than when you using HDD and run out of RAM. Therefore, even though you didn't increase the memory size, but may still effectively make you feel that the RAM size is no longer that important.