Zenith from a practical standpoint.
Performance around the poles depends mostly on polar alignment - the pole is basically the 'inaccessible' point in an equatorial mount because if anything is off, the correction rates go to infinity as you get closer and closer to the pole. Math to do pointing corrections has divide by zero type terms at the pole so correction SW usually has some turn-off or clamping of correction terms within a degree or so of the pole. Field rotation also becomes extreme at the pole if there is any polar misalignment. But that bad patch of sky isn't that big in terms of square degrees on the celestial sphere, and it's not very interesting, unlike zenith.
The celestial equator is the orientation of maximum rotational inertia which is a bigger load versus the pole which is the minimal inertial load, but this probably isn't a big effect on smaller mounts.
When you say test, what do you mean testing for? Slewing performance? Tracking accuracy? Ability to handle imbalanced loads? Wind gust resistance? Pointing accuracy? stiffness?
Edited by 555aaa, 26 October 2020 - 03:18 PM.