そうですね shows agreement. そうですか (1) shows that you learned something. (2) Can also be used to confirm as a reply to itself. そうでしたか shows you had forgotten somenthing. そうっか very informal version of そうですか or そうでしたか. Agreement: - このケーキはおいしいですね。 - そうですね。おいしいです。 Learning: - これ、やめてください! あぶないですよ! - あ？そうですか？ Confirmation by repetition - 田中さんは、四ヵ国語はなしますよ。 - そうですか？ - そうですか。 Regular confirmation - 田中さんは、四ヵ国語はなしますよ。 - そうですか？ - そうですよ。 Informal forgetting - 今どこにいるの？みんな待っているよ! - そっか! 今日だったっけ! (that was today, wasn't it?!)

そうですね is sometimes used when thinking about something for the person whom speaker talking to. ex) Imagine that you are in the real estate office to find a flat, and the dealer would say as paging a file to find the nice flat for you. そうですね。。。この部屋はどうですか？(how about this flat?) そっか is usually used to talk to speaker self when he got an idea for the matter which he is struggling to figure out. Imagine that a husband is alone and thinking about why his wife is so angry... 'Why my honey is so angry to me.... そっか！　Yesteday was our wedding anniversary! ' そっか is categorized into such interjections as Oh!, alas!, dear me! in its usage. And そうですね plays the role to make your conversation smoother. Of course there are not all for the usage of そうですか and そっか. =)

After talking with my boyfriend I've come to the basic understanding that it's mostly just a difference in formality. そうですね is more polite, but he said they can usually be used interchangeably. He's not the best at translating explanaitions into English so I can't guarantee the accuracy but I hope it helps a little bit?