A fixed blade offers two advantages over a folder, if your local laws permit its carry. The fixed blade is ready for immediate use upon drawing. This is an advantage when every split second counts. Also, its design is stronger than a folder. While these points obviously favor a fixed blade, skill and determination are more important factors.
With this in mind, I recommend the carrying the largest blade that is legal in your particular jurisdiction, if you can bring yourself to use a knife. Please remember that any knife is not a "light saber" that can incapacitate a BG with the flick of a wrist. You must close with your assailant and be prepared to inflict gory, potentially fatal, injuries. Think about this before you decide upon a blade for self-defense. Sadly, too many people neglect this awful truth.
Timthinker