I couldn't resist 'doing the numbers' on this.
1. A square shaped base will resist
overturning 73% better than a triangular shaped base, assuming the distance between feet is the same in both cases and ignoring the self weight of the stands. However;
2. It's physically impossible for a triangular base to 'rock' on its feet like a square base will if improperly levelled, hence the reason a triangular base is considered to be more
stable than a square base;
3. In practice, point 1, which probably seems dreadful, will be a non-issue if you're careful and point 2 is quite handy a natural property of a triangular footprint to have.