Spoiler for spoilerish comparisons between CG and Touch:
After having read only 1 volume plus 2 chapters, I would say Kou and Tatsu are quite different. Tatsu is much more socially adept than Kou, for one, and a much less responsible boy. This is due in part, no doubt, to the fact that his own "Wakaba Event" (yes, I know about it) hasn't happened yet, whereas Kou had already lived with his for four years by this point in his life. I can of course see the similarities too, as I can between all the pillars of the story and even the supporting cast. This is Adachi after all, and no one is more internally derivative than he is. That's probably the single biggest legitimate knock against him as a writer, but it's loads of fun to see how he takes superficially similar characters and plots and subtly paints their differences. Because no one does subtle like Adachi...