when your website grows with huge traffic and you need to upgrade your VPS to higher levels. what will you consider to add to your VPS, RAM or CPU? why? what is more important?
A server that is powering a website and databases will be more RAM hungry. But this not something possible to give a direct answer to.
Each upgrade situation is different, you need to monitor what is getting close to 75% usage and upgrade accordingly, but like I said more RAM never hurts.
Although it tends to be the RAM which needs upgrading most of the time, it is still not the case all the time.
Always look at your stats before upgrading. Your VPS provider should have provided you with a control panel which you can view how much resources are being used.