A hard shank tap shoe is not as comfortable as a flexible shank tap shoe, because hard shank tap shoes don't bend or move in the middle section of the shoe. Instead, this part of the shoe remains firm and straight, thereby forcing the foot to move a certain way and limiting the range of motion of the foot. A flexible shank is just that; it bends and moves with the arch of the foot, creating a more comfortable experience for the dancer.
It can be argued that a hard shank tap shoe supports the foot better than a flexible shank tap shoe. This is due to the fact that the foot is supported and bound by the solid nature of the hard shank. When you wear a tap shoe with a hard shank, the toes and the heels are always supported and straight. Flexible shank shoes, on the other hand, give way for you to bend the foot too much -- or overextend the toes -- during an ensemble, thereby missing the solid tap sound.
Various styles of dance can be combined to create a fluid motion onstage, such as tap and ballet, tap and jazz, tap and hip hop, and tap and modern dance styles. Depending on the type of dance you are doing will impact whether you want to use tap shoes with a hard or flexible shank. A straight tap dance, or a tap dance that incorporates some hip hop, will be effective with a hard shank shoe. If you plan on throwing in some ballet, jazz or modern dance movements, however, you might consider using a flexible shank so that the foot can bend and point more easily, which is characteristic of those types of dance genres.
The heel style of the tap shoe often determines whether a hard shank will be inserted into the shoe. For instance, tap shoes with high heels in the back come with hard shanks so that the arch of the foot is supported and the dancer in the heels does not topple over from becoming unsteady. Flat tap shoes or tap shoes with slight heels are more appropriate and suitable for having flexible shanks.