Would it be better to buy teleconverter or 80-400mm lens?

Right now I have a 70-300mm 5.6 and 70-200mm 2.8. I want as much reach as possible, so should i put a 2x teleconverter ($600) on my 2.8 or should i buy a 80-400mm 5.6? ($1500)
I'm using nikon d90

I would go with the teleconverter and the 70-200mm you will still be able to use the 70-300mm in some situations. If your wanting to use the 80-400mm from time to time consider renting one.

You have to realize that by using a teleconverter, you are introducing more lens elements in the light pathway and that lowers the quality as well as using 2x converter will introduce higher f-stops. Your minimum f5.6 becomes f11 and the f2.8 becomes an f5.6, making viewing through the viewfinder somewhat harder as it becomes darker.