The difference between RTA and ICRA is in the complexity. ICRA requires a complex customized tag for each domain. RTA is a simple meta tag that is the same for every site you use it on, and you don't need to register or fill in any forms to use the tag.
Code:
<meta name="RATING" content="RTA-5042-1996-1400-1577-RTA">
Now, you could argue that the Rating Meta is already in place (content="restricted"), as defined by W3C years ago, but it's not consistantly implemented.
I plan to start adding the RTA meta to all of my pages, and the RTA logo to all of my warning pages. The existing rating meta and ICRA tags will remain in place as well.