Solus is a much smaller and independent distro that aims for a cleaner and more streamline approach to software managing and desktop use. So it has less software in the repos, but makes up for it by having the most popular and need to have software. Solus also uses their own in-house DE called "Budgie," that just runs and looks amazing (so does Cinnamon, but I prefer Budgie more)
The best main difference is more so the dev and community interaction. The devs are always taking advice from the community and always looking to improve through the community. I've actually suggested several pieces of software that have made it to the repos after I asked about them. It's something you don't find with most distros and it makes Solus a very welcoming destro for those who want that kind interaction with the devs.