Saplings because juvenile wood is more elastic, at the expense of being less rigid.
Juvenile wood has a higher microfibril angle in its wood fibers (just like compression wood), allowing more bending before breaking (or set).
Since wood is a lot weaker in compression perpendicular to the grain, knots are generally not a good idea. However, in some softwoods (like larch, yew, oregon pine, ...), wood in branches is much denser (and therefore darker), which compensates to some extent for the loss of compression resistance along knots.