For very long walks or long hours on your feet, i found that my crocs flats are more comfy than my melissas. I dont get any blisters at all with my melissas but since walang insole padding ang melissa, masakit sa talampakan pag matagal nakatayo. Mas malambot ang crocs.
Durability - i dont know yet. I only have a pair of crocs. They're 2 years old but i've worn them max 5x pa lang kasi they dont look nice on me pero recently na-discover ko na comfy pala.
My oldest pair of melissas is 5 years old worn many, many times already pero buhay pa. Normal wear and tear lang (like may scratches na ang melflex, di na makintab).
Grendha vs melissa - not much difference in terms of comfort. Durability so far pareho lang din but i only have 1 pair of grendhas.