In switching applications there are more to it than just the Rds-on.

The gate of the MOSFET looks like a capacitor to the circuit driving it. To turn the MOSFET on you need to "charge" the gate. The longer it takes to charge the gate the more switching loss you'll have, the MOSFET is in the linear region for a longer period of time.

To charge the gate quickly you want a MOSFET with low gate charge and/or a driver that can charge/discharge it quickly - that is pump current into and out of the gate. You're limitied to 10mA current at 5V which really isn't much in the scheme of things.

If you want it efficient, don't use a PWM frequency higher than needed and look for a MOSFET with low gate charge (small "capacitor"). Usually, the "larger" the MOSFET is the higher the gate charge is so don't use a 100A MOSFET for a 3A load just because the 100A one has 5mOhm less Rds-on.

/Henrik.