The ATI vs. nVidia thing is a flavor of the month decision, imho. The pendulum swings back and forth, and it just depends on the timing of your personal hardware upgrade cycle as to which manufacturer holds the current lead.
Four years ago I was in the market for a comp, and nVidia was kicking ATI's derriere. Now it seems the roles have reversed. Two years from now, who knows?
So, play the hot hand and cross your fingers it lasts.