Is one better than the other? I see Gibson always talks nitrocellulose on their instruments. And when Epiphone makes a competing version of a Gibson model, they use Polyurethane.

Different advantages/disadvantages.  Nitro tends to be thinner and touched up more easily.  Nitro requires more coats/sanding/time/labor.  Poly tends to be thicker, quicker, and more durable.  A thinner finish generally means a lighter body with better sound.

 The reason they are both popular is that they are each better at certain things. Poly finishes are cheaper to apply so its' better, cost-wise. It's also more durable. And, doesn't discolor. However, it does not repair nearly as well. Nitro affords the ability to make invisible finish repairs as it melts into itself. And, some people like the way nitro discolors in time. Same with the way it checks. Sound-wise, the builder can do things to negate sound differences and I think that aspect is largely just marketing. Choose a finish based on your needs. I love the clean look of a new poly finish. You can't beat that. But, they rarely stay that way and I think nitro looks better as it ages/wears.

 I personally prefer shellac or varnish but they are extremely labor intensive (expensive) and not very durable. As I said, there's a reason that there's so many viable finishing options.