H-bridge MOSFET voltage drop

Don't use resistors!!! There are gate drivers for that. Driving a gate takes sometimea 5A, definitely must be done quickly, not through RC filter (C is the gate). Otherwise you can't do PWM and if you just switch, you risk burning a MOSFET by heat.

You are correct about the voltage drops in the mosfets. The only other major factor would be your supply (Vdd) and its ability to source the current without dropping (e.g. if the wire between the bridge and supply is long and or small). The other thing to pay attention to when using the Rds(On) from the datasheet is the gate voltage (Vgs) that Rds(On) is specified at. If the Vgs is sufficiently different (i.e. the Vgs = 10V and you're going to drive it with 5V) then you should look at the curves to see roughly what Rds you'll get for that gate voltage.

Also, most MOSFETS have built in diodes (called a body diode) so adding the extra schottky diodes isn't usually necessary.

As you have the schematic now, you will have high switching losses. Things to consider are:

• Gate driving. You do not want the mosfet to stay in linear region with high Rdson for a longer time than needed. Especially if you want to apply PWM on the gate.
  You want to PWM the current, not the gate voltage. Use gate drivers, eg: MC33883.
  The resistors around the gates in your current diagram will slow down the switch time.

• back-EMF of the motor. When you turn off motor current, there will be current coming back out of the motor due the inductance and magnetic properties of the motor. When this current has nowhere to go, it will create a high voltage capable of destroying the mosfets.
  To handle this you could rely on the internal body diode of the mosfet. But if you're dealing with a large motor, additional diodes might be required.
  Note: mosfets fail shorted.

• Heat, the mosfets will become warm or hot. You're looking at 2A with 1V drop, that's 2 Watts. Don't forget the body diode current in the heat calculation. Make sure you do not fry the mosfets.

A benefit with using gate drivers is that they often have charge pumps allowing you to use only N-channels. Which have better Rdson == less heat.