I may be wrong (I am a noob), but I thought that the way to calculate the height of the tire is to take the tire width x the aspect ratio. So the stock height is 195 * 55% = 107.25mm, the 205/55 would be 205 * 55% = 112.75mm, and the 205/50 would be 205 * 50% = 102.5mm

So the difference in height from stock to the 205/55 is 5.5mm higher, and stock to the 205/50 would be 4.75mm lower. So technically the 205/50 would be closer to the stock height, also with a lower aspect ratio the tire will handle better, do to the reduced height in the sidewall (more rigidity).

Is this correct?