I need a stable 16MHz clock for a PIC - constant frequency more important than accurate frequency.
I have the choice of using a 16MHz crystal or a 4MHz crystal with the X4 PLL in the chip.

Which should I choose? Does it make any difference?

 

Much easier to make 4MHz crystal work reliably than 16MHz. Trace lengths and board layout are less critical.

Curious as to exactly which PIC and why you want 16MHz, rather than maximum for (unknown) PIC

 

I would consider using an external oscillator in a can. You can easily get a couple of orders better accuracy and stability than with the uC's oscillator (without tuning).

 

I have used the 4MHz and PLL and it seems to work well enough so far.