Tina, I have the 70/300 for canon and love it. It is my most used lens
I bought a Canon 18-200 lens last fall and it is on my camera all the time. I used to go on shootouts with my 17-85 and one other lens (100mm Macro, 70-200mm or 100-400) and found I was changing lenses all the time. With the 18-200 it covers almost all the ranges and it is a really decent macro lens also.
Getting ready for a shoot out and will probably go with the 18-200 and the 100-400.
But I love the 18-200!
I don't know if thus applies to the Nikon, but with my Canon 30D, all of these lenses except the 100-400 and the 100 Macro are rated as EF-S and cannot ever be used if I ever decided to upgrade to a full size frame/sensor camera module.
Not a problem now and can't ever see it happening!