I think both companies have had issues with there drivers in the past, ATI seem to update there's more often though, but in little steps, were as Nvidia updates tend to be less frequent, but larger
Personally i prefer the Nvidia less frequent, but bigger updates, but this also causes issues when a major problem is found, you have to wait longer for a fix
I think it's true Nvidia do spend a lot on working with developers etc, and there logo's are everywere so i'm guessing they do spend more than ATI here, but ultimately drivers can only do so much and if the base hardware significantly slower, well....
I have had both ATI andf Nvidia cards, my current card is Nvidia and so was my last, my prev before than was ATI, i can't recall having any serious driver issues/running games issues on any of these cards, and bought simply on good single card performance as i dont fancy the complexity of SLI/XFIRE even if it's intergrated on a single PCB