i dont really have a preference either way, but ill tell you my observations.
nvidia is releasing their cards first. ati is answering with slightly more power. at the end of each generation, ati is left with the last and generally most powerful card on a per-GPU basis (meaning single GPU ati card vs single GPU nvidia card). the geforce 7 and x1000 series was a strange match up because we were looking at the 7950 vs the x1950 at the top end, and thats dual GPU vs single GPU which isnt a "fair" comparison. i think if you compared the x1950 to the best single GPU card of the 7 series, ati wins.
another thing to consider is the release dates of cards. seeming as nvidia gets started first, and ati has the top spot in the very end, it depends on what sort of performance lead you want and for how long. for example, you can get an 8800 now and be uncontested for a while, or you can wait until the end when ati brings out the x2950 i suppose it would be and that would keep you on top until the 9800 comes out from nvidia. so the biggest difference here is the time of release, but another side note to that would be that the first one out would be uncontested and therefore unproven in battle (and will assuredly be beaten), whereas the last one out would have the definite crown of the generation (assuming it does in fact beat everything else) and will only be beaten by the next generation of cards.
you know what? i dont even know what my original point was now... i cant even finish this idea train. ive derailed myself...