I still take both private and group lessons, but I prefer private. I enjoy the attention of my instructor solely on my horse and I. The moment we have an issue, or I'm doing something wrong, she sees it and tells me how to correct it. It's also nice to be able to work specifically on what I need work on and not have to worry about what everyone else in the group needs help with.
I like group lessons more for my horse than me. It's good practice for him for at the shows to be ridden with a bunch of different horses. I ride on a show team where all 10 of us have a lesson together. It's a lot of fun, but it can get chaotic. Lol
You get something different out of both.