I've a D40 and want a longer lens than the kit 55-200mm. With the new sigma 120-400 and 150-500 out and about 25% difference in price which would be better fot nature(birding etc)and general use? On my old steam Canon EF I had a 300mm lens but it was slow
Neither of these could be considered a fast lens and since they're both new there aren't even any test reports out yet. If you're primarily interested in bird photos and assuming equal IQ, you can easily find 400mm not enough. I wouldn't pre-order either one until some test reports are out.
There's been a lot of debate about this over at the 3rd Party Lens forum. It would probably be interesting for you to read those threads. Ultimately I think the consensus is just what Gerry said, that nobody knows the answer until the lenses are in people's hands.