SSD replacement and RAM upgrade are both ways to improve the performance of a Mac, but they have different effects:

1. SSD (Solid State Drive) replacement can improve the data access speed of the Mac. SSDs use chips to store data, and compared to traditional mechanical hard drives, they have faster read and write speeds, which can significantly improve the startup speed of the Mac, application running speed, file read and write speed, and more.

2. RAM (Random Access Memory) upgrade can improve the multitasking capabilities of the Mac. RAM is the memory used to temporarily store processes and data, and when the Mac runs multiple applications at the same time, having sufficient RAM can allow the Mac to access and switch between these applications faster, improving the overall performance and response speed of the Mac.

You can check whether you have enough RAM by opening Activity Monitor > Memory > Memory Pressure while you are working:

Red: Not enough
Yellow: May soon not be enough
Green: Enough

If the Memory Pressure is green and your Mac has not replaced the SSD, it is recommended to replace the SSD.

In general, replacing the SSD can improve data access speed, while upgrading RAM can improve multitasking capabilities. The choice of which method to use depends on your needs and budget. If the startup and application running speed of the Mac are slow, it is recommended to replace the SSD; if the Mac slows down or experiences lag when running multiple applications, it is recommended to upgrade the RAM.