synchronous buck converter with different MOSFETs
As one of the comments pointed out it is possible and desirable to use different MOSFETs for some sync buck converters.  Often the duty cycle is low because you're converting from (e.g.) 12V to some much lower voltage like 1.2V.  In that case the duty cycle is 10% (Vout/Vin) and the bottom FET is on 90% of the time.  For that reason you would select a low RDSon device for the bottom FET.  The top FET is only on 10% of the time, but is responsible for most of the switching losses. (The inductor current usually commutates the switch node to just below ground before the bottom FET turns on, making it a ZVS transition.)  So what's important for the top FET is low gate charge and fast switching.  RDSon can be much higher than the bottom FET because it's conducting only 10% of the time.  As the duty cycle changes, these compromises will change accordingly.

There's a method for optimizing these choices called the J/K method (nothing to do with flip-flops.)  You can see it here:

The J/K Method For FET Selection