540 and 550 are motor sizes. They just reflect the length of the can's diameter in mm i think. If they are both rated at the same kv then they will both perform the same assuming all the other factors are the same (battery, ESC)
Turns are used for brushed motors. It is a rating scheme based on the gauge of the wire used for the motor windings. This rating is irrelevant in brushless motors. Many have attempted to convert kv to turns but there is no strict conversion.
Kv rating states how many rpm per volt the motor produces. So it directly relates to how many volts you are feeding the motor and your ESC. Obviously higher KV will give you a faster motor but there are lots of factors that determine how the motor behaves in terms of torque and power consumption.