Although they do differ quite a bit in terms of story content, what Pokemon are available, and various other things, in the end they are basically the same. You still travel around Alola, completing trials, and taking down the villainous teams in your way. Ultra Sun is more like an expansion upon Sun, with more new features, added content, and a somewhat different story. If you want to be the judge of it for yourself though, a full list of every change made to Ultra Sun from Sun can be found here.
Hope this helped!