One factor that influences my climbing solo or in groups is pace. When solo I can "attack" a hill at my pace. When in a group I end up riding at the group pace and that often works to my detriment. Example: Solo I might roll hard into the start of the hill and then pace myself up. In a group, I am with people who like to stop pedaling and coast to the base and then pedal up. My choice is to stay stuck in the group and climb in a way that doesn't suit me or go off the front hard and try to get to the right to allow stronger riders to pass. However, doing that often leads people to think that I'm being a jerk attacking at the base. So, rather than trying to explain that to our typical recreational riders, I tend to ride solo more these days. FWIW, my climbing has improved significantly since I'm riding alone more.