No one can categorically say that 12 GB of RAM is faster than 8 GB of RAM. Your software will not use more RAM than it requires no matter how much RAM you give it. To illustrate: If an application doesn't normally use more than 6 GB of RAM, giving it 12 GB of RAM is not going to make it run faster.
To give an analogy: Is a 12-cylinder car faster than a 4-cylinder car? When they're parked in the driveway, they are the same. When they're driving to the grocery store, they are the same. It's only on the highway where the 12-cylinder car shows itself to be faster.
So in order to decide how much RAM your computer needs, you have to know which applications you typically run together and how much RAM they are likely to use under typical circumstances.