Both types are important. The math is indeed more complicated for square meshes, but you can use the fact that the basis functions (is this the proper English translation of the German "Basisfunktionen"?) on square elements have more degrees of freedom to sometimes get better approximations.
Disclosure: I heard some advanced lectures about FEM intended for students who intended to write a diploma thesis about this topic.
Disclosure: I heard some advanced lectures about FEM intended for students who intended to write a diploma thesis about this topic.