I'd recommend that you borrow or rent the Nikon 2x before spending your hard-earned cash. I've found that the 2x teleconverters often "promise more than they can deliver." I've used them off and on over the years, and never quite been happy with the quality. I suspect that your percentage of "keepers" will be relatively low with any
2x teleconverter, even the Nikon. (I've gotten much better results with the 1.4x.)
You might be better off to save your $ for a longer lens like the 300 f4 AFS or a similar lens.
Hope this helps,