D7100 vs. 1.4x Teleconverter for wildlife photography

I'm primarily a wildlife photographer; favorite lens is the AF-S 300f4 and I shoot with a D90. I've been thinking of buying a teleconverter to use with it, mostly for birding. But with the introduction of the D7100 and its 1.3x extra crop mode (at 15.4MP) it seems that getting a D7100 might provide a far more flexible shooting situation than a 1.4x teleconverter (if I recall correctly anything larger than 1.4 won't autofocus with the D90 which requires f/5.6 at least). I could instantly turn on/off the teleconverter without switching lenses, and not lose a stop of light. I could also crop the image even further and get greater than 1.3x magnification with a crop to 12MP to equal the print size of the D90.

Plus, I've been looking for a good video camera, and the D90's performance is less than stellar. I was considering an NEX 5R due to its focus peaking, but if the video on the D7100 is good enough, I would end up saving $100 compared to an NEX 5R (with 16-50 for portability) and Nikon 1.4 for a total of $1300 vs. the D7100's $1200 base cost.

Any disadvantages that I've overlooked? What would you do?

Thanks,

Jacob