Using 'that' is just a little more focus, like seeing something for the first time and pointing to it.
Using 'it' is just commenting on it, like agreeing with someone.
It's very common to use 'that' when you see/hear something new; That's good news, That's awesome! I'm not happy about that.
If you use 'it', it seems you are already talking about it, or agreeing:
A: That's awesome! B: Yes, it is.
So, it's not really as emphatic..