I say they both need changes. I don't have as much experience with Arc, but from what I've played, it seems like the only "build" for Arc is basically just variations on "get abilities back more and electrocute EVERYTHING." Solar, however, has an objective flaw. Two of its abilities are not at full power without a specific aspect and that aspect directly conflicts with another aspect. It's like the class was designed around Heat Rises. Designing all other options around one option is a blatantly terrible design choice that still boggles my mind that it got through the design team. The options should be building blocks for the player to make their own class. One piece should never be the objective "base" piece that you build off of. They should all piece together fluently and build off each other.