Soul Silver is a pretty good game; it has an active after-game, a good selection of pokemon, and following pokemon! Not to mention that Johto and Kanto are good regions to play. I will point out, though, that if you can play a Gen IV game, then you can also play any of the Gen V games as well, but I can understand why you'd want to play Soul Silver. It's a good game.
Like DNA said, the only real difference between Heart Gold and Soul Silver is in which pokemon you'd have access to. I'd recommend that you look at the list of
version exclusives, and decide whether you'd rather have access to something like Growlithe or Vulpix, Groudon or Kyogre, and the like if you haven't already.
There are a few issues with the games, such as the inability to access a few evolutions like Magnezone and Glaceon. Soul Silver, like every other pre-Gen VI game, is incompatible with Wi-Fi, so you might have trouble evolving some pokemon and the events are completely out of date and unavailable, but those are really the only negatives that I have to say about it.