By how powerful they should be, Gogeta. Going by feats, Vegito.
" Just out of curiosity, is destructive capacity the only way we measure strength and power? If not, how about the Haruhi Suzumiya universe since she seems to be able to alter reality to suit her desires. "Nope. There are lots of things that determine which character would win in a battle against another character other than just destructive power. For example, lets's say there's a character that can destroy the universe but takes about 4 seconds to charge up that much power and also has regular human level strength, durability, and speed and has no mental defenses. Someone with powerful telepathic powers can just mindrape him before he can do anything even if they don't even have enough power to destroy a building. Or let's say there's a character with super speed and strength; this character can just beat down the other character before they can finish charging their universe-busting attack. My examples may not be the best, but I hope you at least get the idea that more destructive capability is never the only reason why a character beats another character.