In both use the word "common" instead of usual. The word order us what makes the difference. Either It is common to spend ..." Or "Spending an afternoon in department stores is common." Or end with "a common pastime." The first is better. The difference is in the subject. In the first spending is the subject so you use ing. In the second, it is the subject so you use the infinitive.

El uso de un infinitivo versus un gerundio se determina por el predicado (verbo, adjetivo, o sustantivo), o por la construcción en la que se encuentra, y no por si se sigue un adjetivo. Los diferentes adjetivos toman diferentes complementos y participan en diferentes construcciones. No existe una "regla fácil". After "usual" we typically say "to": it is usual to eat turkey on Thanksgiving. Por ejemplo, I am happy to read the book. (posiblemente una situación hipotética o futuro) I am happy reading the book. (una experiencia real)

It needs to have an infinitive phrase. And besides English is crazy.