Grana Padano is not too different from Parmigiano, actually they are made in basically the same way with the cows milked twice a day, in the morning and at night. In Parmigiano production, the evening milk is skimmed and then mixed with the morning’s milk, while with Grana Padano, it’s skimmed twice, so it has a lower fat content and is aged slightly less, resulting in a less intense flavour.
The important question: why would I use this over Parmigiano? As it’s less intense, this is usually what I will use when making a seafood pasta, it pairs nicely without being overbearing like Parm. It is also better for snacking IMO, a mouthful of Parm is super intense and will linger forever in your mouth, whereas Grana has a lot of the same attributes but won’t drown out whatever you’re eating (get some of the prosciutto we have on special and snack on them together). If you are making an intensely flavoured pasta, let’s say one with truffles or wild mushrooms, I would use Grana as well.