I chose ATI/AMD.
The truth is, I would accept either of them if the came in at a reasonable price point and had enough juice to cover what I needed.
One of the primary reasons I have leaned towards AMD is drivers (ironic twist). You usually hear people angry about AMD, but my experience is the low mid range Nvidea cards (70 to 120 USD) are not well supported. I never had that issue with AMD. AMD usually tips the scales in price, and recently they have been doing very well on performance.
In short, I would buy a beemer rather than a ferrari. The ferrari might go faster, but speed isn't the only consideration; it makes miserable noise, frying heat, and charges a premium. Go AMD!
1) Price. Performance is nice, but one $700 card can be outperformed to two $250 cards. Why not spend less, have an upgrade path, and still beat out the people who have the "fastest" card on the planet.
2) Nope. Having a proven GPU that is a few months old is no big deal. Even the best of games can run smoothly with slightly older cards. Why pay a price premium for the bleeding edge?
3) I am ambivalent. Some sites use synthetics, which tend not to be accurate reflections of system performance. On the other hand, gaming benchmarks are only useful when that is all you do. Some reviews are worth paying attention to, but the majority have to be sifted for content before they can be trusted.