re: Flank vs. Skirt? Now Hanger.

We used to marinate the flank. Not anymore.
We're using an Angus/Kobe breed from Imperial Wagyu. Ridiculously marbled. Wicked juicy.
And yes cutting across the grain is paramount. Use a double bias cut on it- angle you knife to 45° then rotate it a few degrees and cut thin across the grain.
Cook on high heat and for God's sake please let the meat rest.
Skirt has much more exterior fat and needs to be cooked slow to render the fat.
I think both are suitable for fajitas but pay attention to cooking technique.