I met a couple of club members at the Exposure show this weekend and one, who is really committed to constantly improving his skills, was saying he needed a new lens to shoot a wedding.
He shoots a Nikon DX camera and loves his Sigma 70-200/2.8 but found that after crop factor he wasnt able to do groups. His other lens is the 18-55 kit lens. After talking to a representative at a couple of the booths he had concluded that he needed an 18-200. Its not my money so not my place to tell him that spending money on a lens like this will turn out to be a mistake.
He, like many photographers, is discovering that the kit lens is not cutting it for him. But he does need a quality wide angle lens for group settings. The challenge as we know is having effective depth of field control when going wide.
I made the suggestion that he rent a prime or two, along with a second body rather than investing in a lens that duplicates the range he already has. The Sigma is faster and edge sharper than the kit lens or the 18-200.
One of the challenges of a DX body is that getting wide becomes a problem. Nikon does a really nice 20/2.8 and 24/2.8 When we calculate the effect of the DX sensor that 20mm becomes 30mm and the 24mm becomes 36mm. So if he has a big group, neither really suits. If he had an FX sensor, the 24mm would work well for larger groups without fall over effect.
If he has to stay with a DX sensor, his options become more limited. Presuming that the 18-55 wont do the job, I will suggest either of the Sigma 10-20/3.5 EX zoom or Nikons own 12-24/4 IF-ED.
Theres no simple answer to these kinds of questions. The DX/FX or crop/full frame body choices have a dramatic impact on future agility when it comes to glass. This is why I see more and more serious photographers going with both crop sensor and full frame bodies to provide coverage on the wide and the telephoto.