This is certainly not meant to start a flame war (please, please do not flame me) but your situation seems ideal for the Nikon, either the D2x or the upcoming D200. The equivalent VR (same as IS) lens in the Nikon system would be the 80-400, but because it uses a non-legacy chip, that's an effective range of 120-600 at F4-F5.6. A D200 with this lens would cost under $3000 total.
But then, it's not in your preferred system, and I'm a great believer in preferred systems.
JC