Scarf Starmie is equally Pursuit bait for both Tyranitar and Scizor - especially with how easy it can be to lure an Electric- or Ice-type attack. I think the ground immunity, higher SpA and SpD stats, and better general bulk will help over the course of the match more than what Starmie provides. Flygon is also an unreliable answer to DDMence, as you have to resort to a speed tie and then lock yourself into Outrage.
Scarf starmie 2hko's the common scizor and tyranitar, so they cannot switch in and pursuit it. Latias fails to do this, and so is complete pursuit bait and is more difficult to tell if starmie is scarfed.
You say flygon has a 50% chance, it's acutaly beter than that due to the fact that only about half DD mence run max speed. So that makes it a 75% reliable counter to a DD mence.
Latias however, scizor + tyranitar are on over 50% of teams. Then there is the fact that dracco meteor only has 90 accuracy. This accuracy probably cancels out the few that don't carry pursuit. Making it around 50% reliable, maybe 60%.
Now a 50%/60% chance is not reliable, and is why I would not use scarf latias in the current OU metagame. This is why I suggest starmie, but a shucca berry metagross, could also work.