I agree, a flat iron steak is more tender, and not quite as "stringy"...
But for the most flavor, and moistness, I like a skirt steak, as they are well marbled...
and if you slice it thin, across the grain, it is as tender as Filet Mignon... Fork tender... no need for a knife...

all three take marinades very well, just don't over marinade them...
Or they will take on the taste of the marinade, and you lose that great steak flavor...
I only marinade any of these for about an hour or two... with either Terriaki or Itallian Dressing...
Flat Iron steaks are definately a LOT cheaper ($ wise) than Flank or Skirt Steaks...