Nikon's Nikkor 24-70mm f/2.8 lens is widely recognized as the gold standard for lenses in this range. And it's around $1,800.

The Sigma 24-70mm f/2.8 EX MACRO lens (originally made around 2002) comes with a "screw"-type autofocus drive mechanism. Reviews on the quality of this lens seem to be mixed. Those whose copies did not simply come apart appear to be very pleased with the image quality. Those whose Sigma lenses fell or sprung apart (this appears to have been a non-isolated problem) were understandably less pleased. This lens is generally available for about $570.

There's also a newer Sigma 24-70mm f/2.8 EX HSM lens (originally offered circa late 2008 or early 2009) with their more recent, faster and quieter autofocus drive mechanism. Reviews of this lens have been more positive, with fewer complaints about build quality and no reports I've heard of disintegration. The comments I've seen from owners of this lens indicate that they believe the image quality is on par or nearly on par with the Nikkor. This lens is about $900.

Tamron's 28-75mm lens is obviously a somewhat different range, but I believe it is the closest comparable lens in the Tamron line, and is around $400.

I am leaning toward picking up the Nikkor lens, but struggle to justify paying twice as much as the newer Sigma. (I'm not inclined to consider the Tamron, unless someone is very persuasive). I'm a serious hobbyist considering offering limited (e.g., back-up photographer) services for weddings and other such events.

I do not want to spend $900 for the Sigma lens if I'll soon regret buying it and then have to spend the additional $1800 to get the "right" lens. But if the Sigma is actually as good as some reviewers say it is, then the $900 savings would bring me closer to being able to afford the 14-24 and the 70-200 lenses to complete the set.

I have had difficulty telling which reviews to believe. I've not seen any self-described professionals review the Sigma, which itself is telling. And it's not always easy to tell how much the Sigma reviewers know: they often say they "can't believe the image-quality difference" between, say, a kit lens and the Sigma, but I don't know whether they would notice the likely more subtle differences between the Sigma and the Nikkor, assuming there are any.

More problematic, most reviewers have only used the one lens they're reviewing.

I'd really appreciate hearing from people who have used BOTH the Nikkor 24-70 AND one of the two Sigma 24-70 lenses (especially the newer Sigma) and can share opinions about how those specific two lenses differ. Please also briefly explain your most common uses and your general skill level (e.g., intermediate, serious hobbyist, semi-pro, professional).

I'd prefer NOT to hear from folks who 1) simply hate Sigma in general or have had bad Sigma lenses in the past and assume Sigma's past problems persist, 2) are just Nikkor fanboys, or 3) have used only one of these lenses. I DEFINITELY don't want pontification from folks who haven't used any of these lenses (i.e., no cliches like "you get what you pay for" without direct experience with the lens, please).

Thanks in advance for the help!