I don't see a stark dividing line between them. I teach both from the very beginning. I teach a simple grip escape (actually, the foundation of both escapes and techniques); a simple, direct block; simple strikes; etc. as first work. In grappling, attack and defense are less obviously divided, since a block is also an attachment to the striking limb and a counter is also the beginning of a takedown. When we're talking specifically about strike vs. strike, it's a toss-up. I teach the block first, but only because I want to teach the strikes as a follow-up to the block. They happen in the same class.