Power, space, and budget aside, for loudness, should i get one 15, or 2 12s? what would hit harder?

2 12s have more cone area so there is potential to be louder if the install is right.

and less power compression plus more power handling if comparing same models.

both of them in the right box will hit hard, but the 2-12's will usually win that one.

Is this for a certain install?

well its for sure in the future. but i haven't decided on what kind i'm going to get yet. i was just inquiring