Basically a focus group is just that, a group – brought together to voice their opinions on what you are presenting. Usability testing, on the other hand, is singular. One test at a time.
So which is better? Well, the focus group question provides a group mentality answer. Your group of people, whether it be 3 or 30, will feed off each other. It will become more of a group dynamic, with each person falling into roles (the quiet one, the loud one, the laugher, the sideliner, etc).
Why is this bad? Well…as an example…lets say 15/30 people in your group didn’t understand your navigation bar. Are they likely to speak up while the other half discuss the how and why they dislike the color scheme of your sight? Probably not. They’re most likely going to sit back and listen to the others until they understand what is being talked about. Then maybe they will agree that green would look better than pink.
And there it is. The vital piece of information you needed was passed over. Though it is good to know that the user prefers green to pink, it’s not a make or break discovery. With focus group testing, the loudest voice usually gets the most focus. The quiet ones are never even made visible.
Usability testing allows one user at a time to preview and comment on the effectiveness of your design. There is no pressure to fit in or understand or even to agree or disagree. A single user on their own has the right to say and do whatever comes to mind, and that’s what you want.
While focus groups are good for learning about the abstract qualities of your site – your design, your logo, your name, your competitor, etc. Usability testing will focus your user to the quality of your site – revealing what works for them and, more importantly, what doesn’t.