Lens quandary

For my trip to Tromso in September I want to take a good, long focal length zoom lens with image stabilisation. For the Nikon there are basically two options, the Nikon Nikkor 80-400mm VR and the Sigma 80-400mm OS lenses.

Now, both lenses have there good points and their bad points:

+ It’s guaranteed to work with the D200.
+ It’s has some good reviews.
+ It’s a Nikon lens and hence has good quality control at the factory.
+ It’s not large and bulky for what it does and only weights 1.2Kg.
– It costs £1100.
– It has a slow camera driven AF.
– Some reviews think that the image quality at the long end isn’t as good as they would expect (but others say that it’s good).
– This lens uses a first generation Nikon optical stabilisation system which can get confused.

+ Reviews have consistently given good reports on the optics.
+ The Sigma optical stabilisation is top-notch.
+ The lens only costs about £700.
– There’s no guarantee that a lens I’d buy would have the latest firmware which is compatible with the D200.
– Sigma’s quality control is, to put it mildly, variable. Though as this is one of their “professional” EX brand lenses it’s likely to be OK.
– Although the lens has an internal focusing motor, it’s still slow.
– The lens is big and heavy.. 14″ at full zoom and weighs in at 1.7Kg.

So, which do I go for? Do I save £400 and risk getting a bad copy of the lens or have to send the lens off to have a firmware upgrade or get the Nikon and have to worry about the real image quality at the long end and the possible problems with the first generation optical stabilisation?

Hmm.. decisions, decisions..