How to properly switch between two power sources with a MOSFET?
As Madmanguruman pointed out, the main problem is that if the host board is connected, current will flow through the body diode of the p channel FET into the 3V3_2 output.  Knowing nothing of the 12V to 3.3V switcher, that could be a bad idea as it could cause that circuit to function in some unexpected way.  You are going to at least need bi-directional switch to isolate the two 3.3V lines.  Even with bi-directional switch you will need a pull-up to insure the switch is turned off when the power jack is unplugged on the first simpler design.  It also appears that in the second design, if the power jack is unplugged, that the n and p channel FETs would likely be on which would allow 3V3_1 to be coupled directly into 3V3_1.

Also, what happens if both the host board is present and 12V is applied?  This would be effectively paralleling the two 3.3V lines, and could be a problem for either source.  Some kind of contingency needs to be in place if there is any possibility of both lines being active.

You might be able to use a Schottky diode in each line to isolate with passive switches.  There are also parts that function as idea diodes for this purpose like LTC4413 ( http://cds.linear.com/docs/Datasheet/441312fd.pdf ).