OR-ing power supplies (diode or mosfet)

I know this is a few months old, but just for posterity:

Using two PMOS as the OP suggests will NOT work as he intends. It's a mistake I've made in the past. The FETs will effectively do nothing to prevent one power source from back-feeding into another, as the intrinsic body diode will conduct from the drain to the source when under a reverse bias. See this posting to learn more about the body diode and how/why it is formed.

There are solutions with discretes (which invloves using mosfets with a 4th body terminal), but go ahead and trust me, they aren't very simple, and requires some consideration with circuit layout.

Edit: With the 4 terminal FETs you remove the body diode, so you only need one. I should mention that there is a solution using 3 terminal FETs by using two pmos in series, with their sources tied together. See this anwser and this app note. Google "fet common source switch" for more info.

The alternative is to use a PMIC (power management IC) as others have advised. They sell PMICs that have the functionality of an ideal diode (no voltage drop), and others that are more application specific. It really will simplify your life. They'll abstract away some of the device physics complexities, and handle them much more effectively than could be done using discretes. Checkout the TPS2114 as an example.

There are ICs available to do the whole thing, including the switching MOSFETs, full isolation between inputs, etc.

For example, the PS2115A from TI can autoswitch between two inputs, handle 2A, and is currently available for US$2.15 from DigiKey. Take a look at figure 14 for what you want.

If you want to do it with discretes, you can augment the functionality of your FET arrangement as so:

This will turn on the appropriate FET when the appropriate source is connected.

I imagine that it would be favourable to prioritise the USB source for communications functionality so it takes precedence if both sources are connected by switching the Plug pack source off.