Whether or not the subject matter is in motion I think is one variable amongst others to be considered in the appropriate choice of specs. I don't think it is necessarily determinative one way or another, but I agree that more technical flexibility is helpful for this kind of image. You may still want to use a hi-res camera for stationary subjects - it depends on how much resolution you want at what print size and the extent to which you are comfortable with resampling data.
I think the point I'm trying to make is that one has to be careful with one's own weighted
assessment of a camera when there's any obsessional factor involved. Resolution is perhaps the most obsessional of all camera specs. It's the most easily appreciated aspect of camera performance because, however small the improvement, one can usually see it. If it's not visible at 100% on one's monitor, then it might be at 200%. If it's not visible at 200%, then it might be at 400%. The relevance to the print of such increases is another matter.
The fact is, provided that an increase in pixel count doesn't have any downside, then it has to be a good thing, but by itself it might not be sufficient reason to upgrade. At least, that is what I have found to be the case. When I look back on all my purchases of DSLRs, I find that resolution was only a major issue with my very first purchase, the 6mp Canon D60. The 3mp of the D30 was just too few pixels to persuade me to part with my money, so I waited for the upgrade. Apart from a doubling of pixel count, the D60 had no significant performance advantage over the D30.
However, that has not been the case with subsequent upgrades. With each upgrade, the increase in pixel count has been fairly low on my list of performance considerations. My latest Canon purchase, the 50D has fully 50% more pixels than my 40D which I bought on impulse in Bangkok about a year earlier. That increase in pixel count alone would not have been sufficient reason for me to buy the 50D. In conjunction with the autofocus micro-adjustment feature and the high resolution LCD screen, which is just amazing at 10x magnification with a 400mm lens, that 50% increase in pixel count tipped the balance in favour of my being able to justify a purchase.
If I can justify the purchase of a 50D as an upgrade from the 40D, then surely I can justify the purchase of a 5D2 as an upgrade from the 5D1? Oddly enough, I'm having difficulty doing this, despite all of the obvious advantages of the 5D2. However, the increase in pixel count is only slightly greater, as a percentage, than the increase of the 50D over the 40D. We're looking at about 65% as opposed to 50%.
In terms of lp/mm resolution, those figures represent a 22% increase for the 50D (over the 40D) and a 28% increase for the 5D2 (over the 5D1). But these increases in resolution can only be achieved if there's a corresponding and equal increase in lens resolution
, which there rarely is. Using the same lenses with the 5D2 and 50D as one would with the 5D1 and 40D, one could cut those resolution increases in half. That is, an 11% increase for the 50D and a 14% increase for the 5D2, approximately.
I'm sure you'll agree, the true benefit of those increase in pixel count are now beginning to look a lot less persuasive.
I shan't mention the water resistant issues and the limited functionality of the 5D2 video, which are also cause for hesitation.
When dealing with such relatively small increases in 'system' resolution, the quality of the type of lenses one uses most frequently should surely be a major concern. In my own case, the 3 lenses I have found, over the years, that I use most often are the Sigma 15-30, the Canon 24-105 IS (before that the Canon 28-135 IS), and the Canon 100-400 IS.
The Sigma 15-30 is simply not good enough for the 5D2 at the borders and corners which are bad enough on the 5D1. The 5D2 would just accentuate that poor performance.
The 100-400 IS I use mostly on a cropped format DSLR, such as the 20D or 40D, but in future the 50D. I think there can be no doubt that a 50D image with that lens at 400mm will show superior detail to a 5D2 image cropped to the dimensions of the 50D sensor.
So where does that leave me? Perhaps on my next photographic trip I shall be carrying 3 cameras. The D700 with 14-24, the 5D2 with 24-105, and the 50D with 100-400.