I have extensively driven both the cars here in India and they perform well on highways. But the 6 th gen city has a slight edge at high speeds due to its stiffer suspension, which makes it steering and handling more accurate as compared to Yaris.
Being said that, I have many friends here who have crossed more than a lakh km on Yaris and are more than happy with their choice.
Lastly with the limited amount of safety features available in Yaris in your country you have to be a fool to drive at speeds more than 130 or 140 kmph on highways