It depends on what purpose. Phase can be useful for gettin' a weird, unique tone. It's lower output, sounds a bit thin, and is nasal. The degree of this depends on the distance between the coils being put out of phase when use together.
Series/parallel is my favorite pickup mod. It amkes humbuckers so much more useful and versatile. Series wiring is how they're normally wired. Parallel wiring results in a slightly lower output, and is still hum cancelling. It's brighter, and can be compared to single coil's tone. I prefer a series/parallel switch to a coil split.
What type of guitar is this for?

EDIT: What type of pickups, and what kind of music do you play?
Will says:
- SmarterChild - says:
I don't know if I can help it.

Member #6 of the "I play my guitar as high as Tom Morello does" club
IMHO, S/P > Phase switch.

Out of Phase is more useful, the farther apart the pickups are.
Out of Phase is more useful when the pickups are in series, rather than parallel.
Out of Phase is more useful when it's hum-canceling. (two SCs of the SAME magnetic polarity will hum-cancel when Out of Phase)

A HB will not hum-cancel when the 2 coils are connected Out of Phase.
A HB will hum-cancel when the 2 coils are in-phase, regardless of whether they are in series or parallel.
A HB is normally connected with it's coils in series. Connecting them in parallel makes them brighter, but less powerful.

Most pickup selectors are set up to connect multiple pickups in parallel. Connecting 2 HB in series with each other makes them sound very dark, and very powerful. Unfortunately most people don't know how to do this correctly. It's worth learning about. There's a guy over at GuitarNuts2 named John Hewitt that has a few designs doing just that, with a standard 3-way switch and a DPDT switch.

...anything else you'd like to know?
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