The Sonic MU is definitely in Sonic's favor. Since Sonic is so difficult to land a hit on, this could be a while for someone like Kirby. Moreover, Sonic can KO him with all his smash attacks, f-throw, up-throw, back-air and still back-throw. And last, but not least, recovering below the ledge is way more risky than from above, because Sonic has a lot more edge guarding options below than above from my experiences.
Kirby's advantages do not at least match Sonic's. Kirby could solely bait out Sonic's attacks, attempt to find openings, and punish accordingly to the best of his ability. That even depends on the circumstances in the match. That's all I know so far.
Besides a repeated jab1, if there are other safe options to beat out Sonic's Spin Dash, this could increase our chances at this MU. I agree with Seagull's ratio of 60:40 in Sonic's favor for now.