I think it depends on the legs of the DS and Black/White following the launch of the 3DS. Emerald was launched in Europe a good 6 months after the DS and it saw good sales, however if it had been launched a year or 18 months later I think it would be a completely different story.
Pokemon Grey will IMO be likely released in the second half of 2012 for Japan. Therefore, I think it should go to the 3DS. But I wouldn't be suprised if it doesn't go and stick with the DS.








