average Nikon vs. good Sigma lens?

Hello!

I'm planing to buy a Nikon D40 with a kit lens 15-55/f3.5-5.6 (http://
http://www.foto-oehling.com/cgi-bin/...ct/View/157702)
and one additional lens to cover the range from 55mm up to 200mm.

Now, I have a terrible dilemma between following two lenses:

1. NIKON AF-S DX VR 55-200/4.0-5.6 G IF ED
http://www.foto-oehling.com/cgi-bin/...Product/160114
(http://www.nikonusa.com/template.php?
cat=1&grp=5&productNr=2166)

2. SIGMA AF 50-150/2,8 APO DC EX HSM
http://www.foto-oehling.com/cgi-bin/...Product/156149
(http://www.sigmaphoto.com/lenses/len...l_details.asp?
id=3318&navigator=6)

Price is different, but it is not primer criterion, I wouldn't mind
spending more if I know why I did it.
Other characteristics of the lens (like optical/mechanical/practical
use etc) are what it IS important for me.

If you have some experience with both of these lenses, please help me
with this decision.
Feel free to recommend the third option, if it exists.

Thanks in advance.
Cheers!