Saiborg
So its mostly different in their philosophy, rather than the maps themselves? But yeah i guess there are a few differences in designs like you guys mentioned as well (edited)
solo is more of a broader category, race (usually) is designed to be short and to have good flow, usually just being precise movements either by hook and/or by rocket
whereas solo is open to really anything you can do by yourself. Laser maps, edgejump maps, puzzles, climb maps, and are usually longer than race maps.