It's preference man.
It's not an easy choice, because i love both albums equally, though stylistically they have absolutely nothing in common, Siamese Dream is my second favorite Pumpkins album after Mellon Collie, and Daydream Nation is just miles ahead of every other Sonic Youth album i have heard, which includes Sister, Goo and Dirty.
I'm just more of a Smashing Pumpkins person than a Sonic Youth person, because even SP's most inaccessible material is far much easier to take than Sonic Youth's most inaccessible material, which can be borderline unlistenable, and in this case i just find myself enjoying Mellon Collie more, dipiste it having more filler.
So i vote Smashing Pumpkins, god knows Sonic Youth will win this anyway.