"One guy told me that Motorola controllers are more power full then MCS-51 is it true. "

Incidentally where has that guy gone now ? OK just joking.

But when you say powerfull, the adjective needs definition :

* In a portable equipment a MCU drawing the minimum current to do what you want can be powerfull.

* In a mixed signal set up, an MCU with many on board ADC / DAC can be powerfull.

* For someone needing more than 2 UARTs on the same MCU, the one that offers it is powerfull.

* For someone looking to a MCU with the fastest clock cycles, the one that offers it is powerfull.

A boxer ( who knows nothing about a PC ) is powerfull in the ring but could be very weak in front of a PC terminal.

Raghu