Hmmm yes, they are somewhat identical, almost exactly, you can use whichever and it will work, both will be understood by the listener. However, the emphasis I think is greater in the first one though (that or this) rather than the second (it).
This is why I think #1 has a greater emphasis than #2: (again, imagine you're holding a picture from the point of meeting)
"That was the first time that I met him/This was the first time that I met him" seem to have more emphasis since you are specifying that THAT or THIS was the first time you met the person, as if you're directly pointing to the event, memory, or the picture you are holding. It's emphasizing that THIS/THAT was the specific point in time, the very moment the event happened just as the camera took the picture at the very same second.
"It was the first time that I met him" is more of a general statement, with less emphasis. Just like if you did have the picture in your hand but you're not even really paying attention to it. Someone asks you what's on the photo and you just shrug "oh, it was just the first time we met...eh" so not as much emphasis as the first.
(Sorry I had to put it here, the comments box compressed my organized paragraphs into one big confusing paragraph)