A layout flyaway is technically simpler (though not necessarily easier), and reinforces better technique.
As far as technique goes, a layout flyaway is arguably the simplest skill in the entire sport of gymnastics. Assuming you have a strong tap swing, all you have to do is let go of the bar, and the layout pretty much takes care of itself.
A tucked flyaway, while requiring less power, is much more technically complex and must be executed with much greater precision. There is much more that can go wrong. There is the possibility of tucking too early, of closing the shoulders and throwing the head back on release, of tucking too much, of not tucking enough, etc, etc, etc. It tends to reinforce bad habits, by allowing the gymnast to pull in to the bar and bend the knees in the swing. It can be done with little to no tap, which is an extremely hard habit to fix.
A tucked flyaway is, to me, a fall-back option for girls who can't get a strong enough swing for layouts.