I think part of it is some people have different ideas about what is good, and they take those ideas and use them in their "spin". There are positives and negatives about both companies, but often it's how you think about things which dictates which "side" you support.
Some people like NV because they push the boundaries of performance (which is currently coming at a cost of availability, heat, etc) Other people use the negative consequences of this to attack NV.
Some people like NV because they get things done (PhysX, 3D Vision, etc.) Other people use this as a negative because while NV are getting things done and adding these features, they are typically doing it in a closed way. But they are still getting it done.
Some people like ATI because they manage to improve value for the consumer (from a gaming performance standpoint) by having cheaper products (e.g. HD5850, HD5870) which perform very well, or by doing things which cause lower prices across the board (e.g. the HD48xx launch). While they may be a great value prospect, this is also sometimes a negative because they can't take leadership, or require things like dual GPU cards (which come with associated driver/compatibility issues) which mean they aren't always good.
Some people like ATI because they seem to be more consumer friendly, in terms of claiming support for open standards. (things like 3D hooks in the drivers, support for OpenCL/Bullet physics etc). Others see this negatively because they talk the talk, but not a lot actually comes from it. So while it's nice in terms of idealism, the evidence shows it doesn't really do much for the consumer (e.g. there are no hardware Bullet Physics using games and Havok OpenCL died a death).
And then you just have the die hard fanboys who support one company because that's what they do.
Personally I like value and open standards (or I would like open standards if people used them, but I definitely don't like being locked in). As a result currently ATI is the more favourable 'side', but that can always change, depending on who offers the best value, and how support for various technologies pans out.
Someone like Keys (who has an overclocked i7, SLI GTX480 and 8GB RAM) might prefer the company which offers the most absolute performance, whoever that company might be (I hope I am not putting words into his mouth, but he seems to care more for performance based on his posts and his computer spec, and many people attack him for supporting NV).
Just because someone spins something a particular way doesn't mean they are doing it blindly, it just might mean they have different priorities to another person who might interpret/spin the same story in a different way.
From my point of view, low availability of the GTX4xx cards is a bad thing, because in the UK the pricing is already terrible (GTX470 is equal to an HD5870 in price, but with worse availability), so it means that there is no pressure on ATI to reduce prices (I wouldn't buy anything but an HD5850 due to power issues), so this hurts the prospect of any card from either side becoming better value through a lower price. Which is bad because I like value for money.