jmmtn4aj , u mentioned that 70-300 may not have better color than 18-200? Where did u get this info from???
I have 70-300VR and I can tell you that this is an EXCELLENT lens for the price you are paying. VR works very well. This is extrememly impt for a long zoom lens. You can get very sharp image between 70-250mm. If you get a good copy, you can also get sharp images at 300mm.
The VR allows me to show at 1/15s handheld. Try to do this with the sigma.
Did you notice that it is hard to find 70-300VR in the B&S column? Why? Because those who own will not bear to part with it. There is no "alternative" to this lense for the quality vs. price, for those who don't like to carry tripod around. 80-200/2.8 is too heavy and it is also expensive.
Since you already have 18-135, i HIGHLY RECOMMEND you to buy 70-300mm.
I don't own 18-200, if someone who owns both lens is able to give a comparison of photo taken by 18-200 and 70-300 VR at 70mm would be great. I am pretty sure both are comparable, if not 70-300 will be better.