Having tested the A900 and D700, if low light good detail and speed is your criteria (which it would be) - D3. For good light, low ISO uber-detail work - A900 with Zeiss glass - or D3X/D700x if budget permits. Ideally a two-camera combo would do the trick. An A900, even with SSS giving a two-stop advantage, in the situation your talking about is NOT the best choice. A generation or two form now, you never know.
Are you talking about the test you did in the Sony store? The one with less than ideal lenses (no Zeiss or G type optics)? Did you happen to have a D3 there with you to compare?
I have both cameras (D3 or D700, and a A900s) with me on the job when shooting weddings. Side-by-side, shooting the same subject in the same light only seconds between each other, hundreds of shots, lots of weddings.
So far, I can offer this opinion: The high ISO of the A900 is generally underrated, and the high ISO of the D3/D700 is bit overstated on the internet by users. Now this opinion is based on intended use of cameras of this type, usually in less than ideal lighting conditions, not shooting tests in controlled conditions.
The Sony is a relatively new camera and we all are still learning how to get the most out of it. Right now, I've gotten to the point that I can use ISO 1250 to good effect, where when I first got the camera I thought it to be a ISO 640-800 max camera.
So, if shooting stage work, I think the A900 would do very well using the Zeiss 85/1.4, 135/1.8 and Sony 50/1.4 ... all of which are Image Stabilized where equivalent Canon/Nikon primes are not. Depends on how much action has to be captured on stage and how good one is at timing their shots.
I would not take the A900 on the job if I was a sport shooter ... in the first place it'd be overkill, and it's exactly what the D3 was designed for.
As I understand it, the D3x isn't a high ISO camera either with a native cap of ISO 1600. But I'll refrain from commenting because I have not used this camera on the job, only screwed around with it at my local camera store. I can say it's fast! But at $8,000. ... well ... no!