As a general rule, you will be more stable with a wider stance. You will also have a little bit harder time *starting* turns / spins / etc, but once they are going they will be easier to control.
Also as a general rule, it is better to be a little but "duck" stance. This helps when riding switch, and it is also more ergonomically correct when beending through the knees...which you do a lot of in the park.
But as the GOLDEN RULE, mess around with stance and see what feels best for you.
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