They are different... Often to "say" means you are giving an opinion. It also might suggest that you do not care what others think; you just feel you must communicate your thoughts. To tell something suggests you are giving facts. It might suggests that you think the listeners want to hear what you have to talk about. These things I have said about say and tell are not always the case. They are to commonly used to have restrictions - and they are often interchangeable. But these are differences that you can pick up on often.

The last two sentences can be worded better. When you do that the meaning is not exactly the same. There's more I have to tell [you] about it. I have more to tell [you] about it. Here's a good link: http://www.learnersdictionary.com/search/tell