I agree with Otto and Ben. Staying upstairs was a much better plan because, even if you held up in the cellar, you need to make sure the upstairs is fortified. Being in the cellar doesn’t negate the fact that zombies in the house = very bad! Staying upstairs allows you to monitor the house’s defenses, as well as keep an eye out for escape routes. If the zombies get in the front, you can run to the cellar, or use the back/side doors to escape, or even run upstairs. If you lock yourself in the cellar, sure, maybe only four zombies could work on the door at a time, but once it’s open, they’re in, and there’s nothing you can do. Plus, if they tried hard enough, I’m sure they could have gotten through the window in there as well, so it’s not all that secure.
Cooper died because Ben shot him. His corpse was then nawed on by his daughter and came back. Of course, he got about half a sit up in before Ben shot him in the head, but still. And yes, everyone’s death in that movie was through their own stupidity, although even if they had lived, the swarm most likely would have killed them anyway.
Heading up to the roof sounds like a nice idea, except for the fact that THE ZOMBIES WOULD NEVER LEAVE!!! Even if you managed to make it so the zombies could never get up there, you’d die from starvation/dehydration/exposure. And, as Dawn of the Dead/ and Day of the Dead both displayed, just because the zombies can’t get to you doesn’t mean they’re not going to stick around until they do. The same damn zombies sat outside the mall for WEEKS. Unless fresh meat comes by and lures them away, they’re not going anywhere. And you can’t go with the “Well, eventually people got there the next morning, so they would have been fine” way of thinking either because no one knew there would be help on it’s way.
So, yeah, the first floor is the best bet, mainly because it does offer up the possibility for escape. As the remake demonstrated, it’s not all that hard to get away and outrun them, so any chance to get through the crowd could meet with success. Staying upstairs means you have a better means to observe the defenses, and if it comes time to flee, you’ve got a better shot of making it.