Andy makes a very important point here. Bigger is not better if you have poor quality glass on the camera. If you can afford to by-pass the sigma lens and go for the better optics of the Nikon it could save you money in the long run.

Most of the Sigma lens are made to a price and quality to under cut the prime manufacturers. With lens you do tend to get what you pay for. Unfortunately the good lens are very expensive and it can be a false economy to cut corners if you are striving for quality images. The Sigmas can give very passable results when stopped down to f11 or so.

My first telephoto zoom was a budget Canon 75-300. It was OK until I started to take bird photographs. It gave quite passable results if the light was good enough for a high shutter speed and stop down to f11 or more. In high contrast scenes it would colour fringe badly.

I wanted a better lens without paying a fortune. The Sigma 135-400 seemed to fit the bill although it was not highly recommended. I found a nearly new lens on e-bay. Well it solved the colour fringing problem and it seemed OK on my Canon 300D 6Mp. However, I can't say that the number of keepers increased with this lens. After upgrading the camera to the 20D 8Mp I noticed that the image sharpness was an issue.

So I bit the bullet and managed to source a second hand Canon 300 f4 L. The difference was quiet amazing. The number of keepers increased dramatically.

This evening I was trying my luck at getting inflight shots of Swift over my local lake. I used the last frame to take a snap of this passing aircraft.

Time 8.40 pm - 20D - 300mm IS f8 - 800 ISO - 1/250 sec Hand held.

This is a 100% crop or 1/20 of the image area.

If I had taken this with the Sigma 135-400 @ 400mm the shot would of been blurred and impossible to make out the detail on the tail. ( I was never able to get a decent aircraft flight shot with that lens )

Nikon claim VR technology will allow a photographer to shoot three stops lower. If you can shoot 1/60 then with VR 1/8 (approx) is, in theory possible. In reality if you can shoot aeroplanes at 1/60 then you should still use 1/60 and be confident you would have a greater number of keepers. Panning would be easier as vertical movement is automatically sensed and cancelled out. Sigma produce a version of 80-400VR called 80-400OS. Perhaps you could play the currency game when you are in the United States.

I will look at the prices of kit when i am over there. I was shooting all of Old Warden today at 1/125th so i look forward to seing my results.