I guess it depends on the camp.
I'd normally say lessons because the level of instruction is likely superior.
Although, camps can be really, really good. It depends of course, on the camp, but I remember my first pony club camp was a turning point with my first pony. We'd spent months working on things but after a week of four or five hours of riding a day it all came together and we really started working as a team.
Although this was through the pony club with my own horse, and prior to this I had done camps through the local riding school and they weren't so good. Fun enough but I didn't really learn and progress in the same way. That was just basic flatwork and games, a few trails and jumps, but the pony club camp had really good instructors and we did dressage, showjumping, cross country, polocrosse, sporting and mounted games, along with other unmounted courses in farriery and driving.
So you have to make up your mind for yourself. If you're likely to be one of the best riders in the group you may get a little bored.