On the plus side, at US Nationals, you get to see the Juniors, and if you are willing to cope with the travel to the alternative rinks, the Novices. It is really nice to see the up and coming kids skate. At Worlds, you will see only seniors. However, you get to see the Qualifying rounds, which are not part of US Nationals.
On the down side, at US Nationals, often the politics is as thick and perhaps more so than at Worlds. And the price for seats is sometimes even higher than Worlds.
On the plus side again, at US Nationals, you may get to see Rohene Ward, Angela Nikidinov, and Jane Bugaeva skate. There are always great skaters, that for whatever reason, will seldom make it out of the country. Many of them have a great deal to offer. The likelihood of Rohene making it out the US is very small (but I used to go to Nationals to watch Rudi Galindo skate, and said the same of him...hmmm)
At Worlds, you get to see all of the very, very best. And that is the overiding thing. But when World's are some place that I can't get to, I go to Nationals.
And when I have no money, I go to Skate America, Cheesefests, and maybe Sectionals or Regionals. I am also going to look into going to Jr. Worlds in Kitchener in 2005, but I haven't seen a website where tickets can be ordered yet.