It doesn't really matter...
A List.clear() implementation sets the internal array's references to null. Effectively setting the objects to be garbage collected if there are no more references.
If your only concern is memory there's no real measurable difference in both approaches. Even operation wise the difference will be in array allocations (in resize operations) and other such operations.
But clearing it may be marginally better although if creating a new list is more readable I'd go with that.
