Eh. I see it as a dangerous match up. Marine captain against a relative newbie. The only reason I make Mitsuki out to be superior is because, technically, he outranks them. I'll give him a slight boost. I see their relationship being similar to Luffy and Zoro. Zoro seems to have more raw strength to him, easily the strongest out of the crew in that regard, but Luffy has more drive and learns surprisingly quick for being an idiot. He almost evolves during the battles he has. Picture Mitsuki being a bit stronger for a while, but Ryu can easily surpass him once he gains full control over his powers.