Personally, I would almost never use a PZM or contact transducer in favor of other mic choices. They don't do the resonance of the instrument as a whole the justice it deserves (if it so deserves...)
But...answer these questions and it will help us to understand which direction to point.
1 - Is this for rock, jazz, classical, etc.
2 - Do you want a very forward sound (Elton John) or a very laid back sound (Liang Liang)
3 - What mics and preamps do you have at your disposal
4 - What is the room like? Is it a studio, a living room, a concert hall?
For classical, I almost always use spaced omnis (given that the room is acceptable and it's a solo performance. If it's with orchestra, I'll use whatever I can to isolate it as much as possible while still getting a good/usable tone).
For rock, I place a pair of cardioids inside the instrument over upper and lower strings. Or occassionally I'll place them at the lip aiming in.
For jazz, I'll often start with a pair of ribbons in Blumlein behind the pianist aiming down into the instrument.
Anyway...there's many different ways to skin this cat. The closer we can get to what breed of cat, the better chance we have of getting it right.