I hope you are aware that there is no turbo planned for the US smart. My guess is if you have issues with the Cooper, you will not be happy with the smart. As the MINI has an advantage in power-to-weight ratio, 22 lb/hp vs. 26 lb/hp in the smart. Plus it has 62% more torque for only 40% more weight.
Hill start assist prevents you from rolling backward on a hill, but it won't make you go up the hill any faster.
If you want something tiny and with power to spare to go up hills, you should look into the new Cooper S.