Tokina 28-70mm at-x pro f2.8 or sigma 18-50mm f2.8-4.5 DC OS HSM?

I'm very tempted to sell my Tokina 28-70mm f2.8 lens to get this sigma 18-50mm f2.8-4.5 with Optical Stabilisation. I'm using a Nikon D200 and wants to use a lens that is perfect for all around, can you please help me decide? This tokina 28-70mm f2.8 constant is very soft wide open and doesn't have a VR, I'm planning to use a lens that is good for portrait (I'm wanting to shoot a wedding as well). The reason I'm wanting this sigma 18-50mm f2.8-4.5 is because it has Optical Stabilisation and the aperture is better than the kit lens, now can you tell me if I'm stupid ever thinking of replacing this lens with this new sigma 18-50mm OS?

I will appreciate any opinions you can provide me. I will decide based on your opinions.

I just looked for and got a Tokina 17-50mm f/2.8 non-stabilized lens and love it. Even though I've got a Nikon 18-200mm VR, I've never found a need for stabilization in the wide to normal angle views. But, my main focus (bad pun intended) was to get a low-light lens what would allow me to use a faster shutterspeed in challenging situations.

As HisWife.said, you'll feel the hurt when you zoom out to 50mm on the Sigma and find the world getting darker and your shutterspeed getting slower. Let's put it this way. With that lens, you'll NEED the OS at most weddings. With an f/2.8 lens, you won't.

At those focal lengths optical stabilization is kind of irrelevant. The rule of thumb goes that your shutter speed should be equal to the reciprocal length of your lens. So if your at 70 mm your shutter speed would need to be higher than 1/70 (which doesn't exist so 1/80) to shoot hand held. That is the cut off for hand held anyway and 90% of the time you will use a shutter higher than 1/125. If not, then you are using a tripod and long shutter speed. At some point IS/OS becomes a hindrance and actually has to be turned off or the electronics within it actually CAUSE vibrations in the image.So, the 18-50 is a no brainer. That variable f/ would turn me off right there.
Plus Tokina is the superior maker of the off brands in terms of quality in the optics, build…

That f/4.5 will HURT LIKE HELL for a wedding in a dark catholic church. And if you are trying to get sharpest images out of it? You are at about f/7.1. OUCH>

All lenses are sharpest at about 2 stops down from wide opened, so the soft end of the Tokina doesn't surprise me.
If you really want to replace that lens with something of comparable stats look at the tamron 28-75 f/2.8 or the Sigma 24-70 f/2.8. OR the Sigma 24-70 f/2.8 OS. The 18-50 just isn't worth it over those others. Consider the price… It's all of $200.In lenses it's a pretty good guarantee that you will get what you pay for.

ETA: George's point about needing the OS at wedding with one and not the other is a good one!