They have slightly different nuances. I couldn't say.=I don't know. Example: A: How long will it take to get from London to Paris? B: I couldn't say. I have never made that journey, so I don't know. I wouldn't be able to say.=I don't know because it has not happened yet. Or, I am unwilling to tell you. Example: A: You heard that conversation, so how did it really go? B: I don't think you need to know, so I wouldn't be able to say.