It is hard to give anyone advice when the safety of their data is at stake.
But most of the Reset options, Windows 8 or Factory will give you options as to what you want to reset. I just did one last night where I told it to clean the entire drive and it took several hours to format the 2 TB drive. I had other options, such as just where Windows 8 is installed, and remove only my files. You will have to see what your options are and then watch the messages to see what they might indicate will happen.
Your OEM should have guidance as to how to do a Factory Reset and what the possible outcomes might be.