I know a lot of people have already put in their word, but I just wanted to add.
To get one or the other is dependent on what you intend to use. The Nintendo Wifi USB router is great for Nintendo consoles but that is all it can be used for. General Routers are not as great in getting your DS or Wii a absolute solid connection, but they can be used for several other things, from laptops to...well, a lot of stuff.
The choice of what to get is up to you, but do think about the long run, how useful will a USB router be? That depends on how much you use your Wii and/or DS. But on the other hand, how useful will a Router be? That depends on if you use Laptops and other Wireless internet stuff in your home.