Remember, stores were opening at midnight and newscasts were reporting on MW2's release. The game was incredibly well-hyped and the sales reflect it. However it's likely that the majority of people who are interested in this game have already purchased it, so expect its sales to drop dramatically over the next couple months.
Now as for NSMBWii, it also released to strong sales (not as strong, admittedly), but it will continue to sell for years to come. Long-term, it will outsell MW2 easily.
A similar event happened when MarioKart Wii and GTA4 came out around the same time. MarioKart is still selling in the top 10!