nRF51 or nRF52? MTU Packet is the question!

To pursue a higher speed, performance, and functionality is eternal prospect of technology moving on.

Inevitably, the payoff in term of cost of the the technology improvement is essential.
Nordic has launched various of BLE SoC such as nRF51822, nRF52832, and nRF52840.
Raytac recently has been asked frequently~How we can decide to use nRF51 or nR52?
Which solution is the best adoption for the implement?
Is the best and latest solution always the right option for developers?