SSD vs Memory upgrade
SSD's improve the start-up and shut down speed and will load apps installed on it a bit faster. 
More ram will be very useful if you are running multiple big apps like xCode at the same time.

I recently upgraded my SSD and I have upgraded my ram before that and both made my machine a lot faster, the SSD with booting and the ram with running apps.
It really depends on what you would like to speed up in your mac.
If you are not sure I would recommend more Ram since it is less of a hassle, cheaper and the performance boost is very noticeable especially when developing in Xcode