A solid frame is sturdy and does not allow the flapper ball to twist or misalign with the drain seat. It center punches the drain every time for a perfect seal. The all flexible flappers are great to use when you have an over sized 2 inch flush valve which does not allow a solid frame to fit (it may stick on the drain and not close). The flexible flapper comes equipped with an overflow pipe adaptor which allows you to connect the flapper to the drain when it does not have mounting pins for a flapper to attach to.