I prefer ensure_ascii=False because it's efficient.
But I notice it is very slower.

On Python 3.4.2:
In [3]: %timeit json.dumps([{'hello': 'world'}]*100)
10000 loops, best of 3: 74.8 µs per loop

In [4]: %timeit json.dumps([{'hello': 'world'}]*100, ensure_ascii=False)
1000 loops, best of 3: 259 µs per loop

On Python HEAD with attached patch:
In [2]: %timeit json.dumps([{'hello': 'world'}]*100)
10000 loops, best of 3: 80.8 µs per loop

In [3]: %timeit json.dumps([{'hello': 'world'}]*100, ensure_ascii=False)
10000 loops, best of 3: 80.4 µs per loop