Faced with a choice between those two, I'd go with the Sigma. The Nikon will soon be replaced by a current-generation lens with IS, and might be a pretty poor investment once the new model hits the street.
Sure, Sigma lenses don't hold their value well either, but you are £1000 in front to start with.
Quite a few people use the Sigma 500/4.5, and it gets good reports. Me, I'd either get the superb Canon 500/4 and swap bodies (cost you a lot less to do that than you'll be spending on any of these lenses), or else wait till the grossly over-priced but sure-to-be-wonderful VR Nikon arrives.
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