As a participant, I prefer a group dynamic. It’s the energy, the cameraderie, the variety, the interaction. I would rather count the hairs on my cat than do weight machines by myself, and I know how to modify my own movements up or down to get the workout that I need from that hour.
As an instructor and trainer, my favorite is the small group. There’s still the energy and group feel, and it really helps retention when I assign people workout partners and they’re accountable to each other. With the smaller group, I can focus more on individual needs than group needs, so clients get a more tailored workout than they would in a large-group class.