Theoretically, it doesn't matter, especially with the short lengths of speaker cable we typically use. Both methods put the two cabs in parallel as far as the amp sees it.
But the finicky purist might argue that the signal that runs to the second cab (when daisy chained from the first) takes a wee bit longer to travel, because of the longer distance along the circuit, and thus there's the slightest bit of time alignment delay or phase shift between the two cabs (I mean really imperceptible). Just out of habit (due to some deep subconcious commitment to equality -- I'm a Libra) I've always done the separate connection from the head for each cab. This also has the benefit of one cab not relying on the other's single connection in case that cable or a connector fails. But it's a bit of a toss-up.
