The problem with saiyans is that there power lasts for as long as they have energy. Guardians are undead, they wouldn't tire out. If Goku tried to use the spirit bomb, he'd be nova bombed, fisted, golden gunned, and gjallarhorned into infinity before he could charge it for launching. That and Guardians don't die unless their light is destroyed. Something of darkness would be able to actually destroy us. (Correct me if I'm wrong but the only race that can kill us is the hive since they can eat our light) In conclusion, Goku and Vegeta will either lose, or end in a stalemate.