Both are fine oils- you can't go wrong with either.
Don't let the "Semi-Synth" fool you into thinking Motorcraft is a better oil though, PYB's Group II+ base oil is just as good as a 25% Group III/75% Group II combo - all else being equal.
I have considered trying Motorcraft several times, but to date have stuck with PYB.
I just switched my girlfriends car from PYB 5W20 to MC two days ago. I think both are great oils but the MC in the 5 Qt jug for $11.50 everyday price at WM is hard to pass up.
I'm sold on the MC. My Grand Marquis loves it, my analysis from Blackstone confirms that, and the price is hard to beat when purchased from Walmart. Win, win, Win!