How to get the best performance in MBP 13" mid-2012?

If you can afford it, replace both. Doubling RAM to 8 Gb already makes you feel quite a difference with daily use tasks and increases the amount of Vram available. Going up to 16Gb in my humble opinion gives you a marginal benefit and would mostly make a difference just in case of handling large files with demanding Apps such as photo, music and video editing or gaming. SSD prices have dropped significantly as of late and considering the about 4 times faster read speeds a replacement would be a must for a Mac like yours as it carries along less heat production and lower power consumption as bonuses too.

If you replace both at once it would feel like having another Mac from day one. Only make sure your fan and heat sink are always fairly clean to keep temperature under control, that's very important with demanding tasks such as gaming and editing.