"Grandfathers are people."
Translation:Avoj estas homoj.
I just did a quick check by looking at word frequency in two recent Esperanto publications. "Homoj" is more common. It's important to consider the intended meaning, however. It's like asking which is more common in English: human (being) or person?
If you consider the semantic space occupied by "human (being)" and "person", a similar semantic space is occupied by "homo" and "persono". The difference is that "homo" (compared to "human") is a little broader in meaning and use and will be seen in many contexts where we say "person" in English - such as the one in the OP.
The core meaning of "homo" however, is human being - so if you mean human being, you HAVE TO say "homo". If you want to emphasis personhood, that is, the rights of the individual or the division of God into three persons... you have to say "persono."
No, it wouldn’t, esti (“to be”) is not a transitive verb, but a linking verb. It cannot take a direct object, hence no accusative here. It links two parts of the same rank which are both in the nominative.
In connection with a verb the accusative is used only for direct objects. These verbs are called transitive. (Mi vidas lin. – I see him. versus Min vidas li. – He sees me.).
There are intransitive verbs which do not take an direct object. (Mi dormis. – I slept.)
And there are linking verbs as explained. (Mi iĝos kuracisto. – I will become a doctor.)
Because in English only with some pronouns (I/me, we/us, she/her, he/him, they/them) the accusative is marked, it is difficult to see it in English examples. And because of the lazy use (Is it me? instead of Is it I?).