I am not 100% sure about technically which one is capable to reach high speed sooner but when you drive you feel the difference. I have driven both and I am not ready to believe that petrol will beat the diesel in the city as mentioned in the previous posts in this blog. I will accept it if you beat me in city.
About the braking distance:
My understanding is torque is there even when you are not pressing the accelerator (idealing torque) which is higher than petrol. On a practical note, try the braking in both and you will certainly say that petrol's brake is quick and instant compared to diesel. If not torque, there may be some other reason but check that out. The diesel Swift weighs more than the petrol and the brakes in both cars I am sure are the same. So if you think braking distance of D is more than P, it could be due to that alone.