better for what? the only real argument that could be made is in the area of drivers regarding specific game issues (the ati seems to have more, but they both have issues, and not every user experiences even the "common" issues), and then it's more a matter of how willing you are to wait for a patch or find a workaround. however, overall driver stability from both sides has been shown to be pretty decent.
at any rate, it's pretty much been shown over and over that at the same pricepoint, the 9800pro offers better overall performance and a better overall feature set... so what's the point in beating a dead horse? drop the cost of the 5900u about $50 lower than the 9800pro, and you have some interesting arguments... but really, it's pretty clear ati won this round (and the one before), and until the next gen cards are out in a short time, this argument is rather passe'.
either card is a good choice depending on what's most important to the person who is buying it, and from most of the pre-release hype surrounding the nv40, it's a safe assumption that neither of these cards will be anywhere close to the performance of the "next gen" cards, at least if you buy into the hype