best overall card as far as performance/quality/stability/support goes
If you're going to go by this you wouldn't go by ait/nvidia it'd be the companies that sell the cards such as evga, xfx, mis, powercooler, etc etc etc.
That being said I personally like the companies that sell nvidia cards because because most of them seem to be more reliable and are bigger names than the ones that sell ati.
When it comes down to it you should base your choice on a lot of factors, not just the best performing card..because sometimes that card could cost a lot more or isn't even worth it. I've said it a bundle around this forum and I still stick with it, the 4850 and gtx 260 can max any game out right now..so anything above them is kinda a waste of money unless you have a huge screen or want to flex your e-peen.
As I said.. a lot of factors, these would include:
company warranty (some cover overclock, some don't...some cover you changing the fan, some don't..etc)*
upgrade ability (evga is the only one I know of personally)*
video ports (things like do you need hdmi..and then ATI has eyefinity and nvidia has nothing close)*
space requirements (i've seen a lot more single slot ati cards than I have nvidia)*
board type (as far as I know ati has the best AGP card..but who usees those?? lol..and other reasons)
overclocking/adjusting software (I don't care what you say..but evga precision is a lot more user friendly than CCC lol)
(*= falls to the company for responsibility, **= sometimes the company will have a bit lower price than the others..but the same card will usually be around the same cost range..unless it's like evga vs a no name seller..then the price might have a big difference..but I'd still go with evga/known name seller)
That's about all I can think of...but I'm sure there are more factors. But if you want to get right down to the statistics, according to benchmarks the gtx 480 is about 10% faster than the 5870. Along with that 10% performance boost it is also 10% hotter and cost about 9% more ( I haven't done the math to see cost for average fps..so ATI could easily take that) so you have to look at things like that to see if that card is really that much "better".