I have used both ATI and nVidia cards and found the nVidia is the way to go, due to the huge driver issues that ATI have (They are just plain c**p).

For me I would go either tbh, the difference in performance is not going to be noticed by you, but your PC may note a slight difference. Remeber these tests are very specific and they are done in near perfect conditions and equipment. a nanoSecond here or a mHz there is not going to make an ounce of difference to you. Only the price and the basic specs of the card matter. I currently use nVidia, but theres nowt wrong with an ATI card, until the catalyst drivers get installed lol.