Nikkor 18-70 vs Sigma 18-125/200?

[Skylined]

Body:
D70S (without Kit)

Lens: all Nikkors
50mm f/1.8
75-300 f/4.5-5.6

I'm looking for my next lens, which has to be on the wide angle side, since I cannot work indoors and do studio shooting with these two lenses that I have right now.
I'm going to sell the 75-300 to get some money to buy the new lens.


First of all let me tell you that I'm from Uruguay, South America.
I can only buy stuff in USA whenever somebody that I know is traveling there, so he/she can bring it back to me.
I cannot ship stuff to my house because customs charge me 80% over the cost and s&h, and the "gift" trick doesn't work.
Talking about money, the Nikkor 18-200 VR is totally out of contest, it's way to expensive for me.


There's a mate that has another D70S and a 18-70, so I already know how this lens performs.


I've read many things about the Sigma 18-125 and 18-200 and I'm a little bit confused.

18-70 vs 18-125
- I know the Nikkor has a faster AF, but I would like to know how slow is the Sigma, can it be compared to the 50mm's speed?
- Which one gives sharper images?
- Which one gives better colours?
- I don't want to sacrifice image quality for having just a little bit more range.
- I don't care about focusing noise.
- some say that some of them have a focus problem, you can either get a good sample or a bad one.
- some say that the bad samples are just the ones for Canon, what about the ones for Nikon?


200 better than 125:
Some people say that it's better, but...
- I've seen some photographs with more CA.
- It's softer.
- More on the tele side, but some say that it's really a 155mm
- More expensive


In case I'm getting the lens over here, the 18-200 is discarded due to its high price (200 US$ more), but if I can get somebody to bring it to me from USA, I'm going to take it into consideration.


Prices here
Nikkor 18-70 - 400 US$
Sigma 18-125 - 400 US$
Sigma 18-200 - 600 US$


Any help and comments are more than welcome.