It depends on what you mean by “better.” If you are only results driven, then I would say that 80% of the time they’ll get better results with one on one training. I’ve had a few pairs of clients who are equally matched fitness-wise, however, and their pushing each other makes them both better with great results, so I could argue that a well-matched pair could get close to the same results if not better.
Once we start talking about groups of 4-6 people (my standard group size), the “better” comes into play because of the affordability and the cameraderie.
What I’ve done to get better results for small-group is to combine one on one and small-group so that individuals can see me once a week and do small group once a week so that they learn about themselves, how hard they can work, what modifications they need, etc. during one on one, then they can push harder in small-group.