I don't think either will move a significant amount of hardware, maybe a 10% bump at best. While both look to be pretty good, the amount of people interested in either title, who don't have the system already is very small since both have current gen iterations that have already been released.
In terms of moving software Halo: ODST should easily win, but that's not to say that Uncharted 2 will do poorly. Due to the general esteem in which the first game is held the sequel should easily sell more because some people who only rented or bought the first pre-owned will buy it. That will add to the frontloading of the title, but large initial sales are always good because they attract positive attention.
Besides that, large openings and strong legs are not mutually exclusive. Halo 3 had a launch week of nearly 4 million, but has added almost an additional 6 million since then.