Focus Sash only makes sense when you pair it with a glass cannon. Basically something that usually doesn't take a hit well, but still needs ways to boost itself. For example, you could give a Focus Sash to Minior which can take that extra turn to set up a Shell Smash allowing it to wreak havoc on your opponent.
The Mega Stone should usually go to your main damage dealer. The stuff that either gets an immense boost in its offenses, or enough of a boost in its defenses to set up and wreak havoc. Popular examples are Salamence and Kangaskhan who are just incredibly broken really good in those battle facilities.
Imo Aerodactly doesn't really fall under any of those categories. It doesn't really have a good boosting move and its offensive capabilities in Mega form aren't that great either. Maybe a Life Orb or a Choice Band might be a better choice, considering that it is pretty speedy by itself. Just be aware that it doesn't really like getting hit.