I've been suffering from the same quandary for a while now, considering investing the money or buying a 360 - though I'm a little confused; you'd think they'd have fixed the 360's problems by now on the new units.
Anyway, I'd say get House of the Dead: Overkill and/or Madworld to tide you over and see where the Wii library heads in another 6-12 months. If it turns out to be a flood of casual games without a hardcore game in sight, you're lost. Sell it to a 12-year-old or a mom and exchange the non-casual games at your local game retailer.
If you're just looking for as many hardcore games as you can find, yes, hock the Wii and buy a 360. However, note that many of the few hardcore games that were released for the Wii have been at least decent, if not better. Twilight Princess, HotD: Overkill, Madworld, No More Heroes, those are all pretty good games, while there's a deluge of games that are all right, just not great, on the other systems with a few gritty gems hidden in the grit, there are just some more gems there than on the Wii. With the Wii, at least you tend to know where the good games are (with the possible exception of Redsteel <.< ).