Hey Xanthus,
The P-Megaswitch emulates the standard PRS pickup selection options, so you can always Google a bit to find out what people say about them.
When you split a humbucker and use just one of the coils, you will sort of "halve" the output... as well as restore some higher frequencies. That's going to get you sort of into Strat territory; however, there are other differences in construction (bar magnet vs. separate magnet polepieces, different wire guage and # of wraps) that make a split coil not quite the same. Some manufacturers have made humbuckers that will split into a "true" single-coil, but your PAF isn't one of them.
Series vs. Parallel is easy to understand once you sit back and think about what the words mean. Two coils/pickups wired in "series" are ganged one after another. Coils/pickups that are "parallel" each generate their own output, which gets "blended" before output. Humbuckers in "full humbucker" mode are two coils wired in series, which is why the resistance (inductance? I'm not quite knowledgable enough to pull off the right terminology and I don't want to look it up. ) is higher and consequently the output is higher. By contrast, when two pickups are wired parallel, the resistance/inductance of one coil isn't added into the next... meaning that the actual resistance/inductance (whichever is correct... both?) isn't doubled.
End result in terms of the P-switch: The "series" option is in a class all its own-- strats don't have the outside coils in series without wiring modifications. It'll be a bit fuller than any strat sound, but also different than a single humbucker because of the locations of the 2 coils. The 2 parallel options won't be THAT different from one-another-- using the inside coils and you'll get a sound *sort of* like the in-between "hollow" sound that so many strat lovers enjoy. The advantage over the E-switch is that all positions are humbucking. This "disadvantage" (and IMO not really that bad) is that you don't get a true single-coil strat sound in any of the positions.
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Or the short version: don't worry about the names "parallel" or "series". Just know that you'll get your full humbuckers, plus the ability to emulate lower-powered strat "in-between" sounds, while retaining humbucking performance in all positions. Add the unique "outside coil series" option, and I really think it's a better option than the E-switch, but that's just my personal opinion.
Greg