Firstly a sensor with twice the pixel count would be equivalent to a 1.4x teleconverter, in terms of the number pixels on your subject so a 2x would give you more detail. But lets keep things equal and consider the case of a 1.4x TC.

Fundamentally the advantages of cropping are flexibility and less gear to carry/swap. You can correct the composition, or decide you prefer a wider shot, neither of which are possible with a TC. Teleconverters also slow down AF performance, by quite a lot in the case of a 2x.

Fundamentally the same process is going on in both cases - the TC spreads out the image, the cropped high resolution sensor finely samples a small region of the sensor. Thus a good quality TC and well matched lens should be able to match the quality you get (or thereabouts, the lens elements in the TC will impart some aberrations).

This equivalence also applies to light gathering. It's true you lose a stop of light with a TC, however the higher resolution sensor will have smaller pixels which gather less light, in fact the amount of light from your subject falling on the sensor is the same in both cases. The only advantages when it comes to noise will be from improvements in sensor technology.

The same situation occurs with camera shake, the TC based solution has a longer focal length, but a higher resolution sensor is more sensitive to motion blur, so both systems will behave similarly.

An alternative option altogether is of course buying a longer lens, but I'm afraid I won't use it enough to make it worth it.

In that case have you considered renting a lens? It generally doesn't cost that much for just a couple of weeks (compared to the total cost of the trip) and you can get all the reach you'll ever want (only risk is you wont want to give it back).