Liquid rocket engine's hot fire testings are performed both vertically and horizontally.

Vertical orientation - Falcon heavy, Falcon 9, Rutherford engine, UCSD Engine test from 2014 , Merlin 1D, MASA UMich

Horizontal orientation - UCSD Engine test from 2015, Launcher Space Test, Masten Space system test, Armadilo Aerospace, SpaceX's Raptor

What is the basis of the testing orientation - from the point of view of safety, size, complexity (or ease of operation), data collected, etc.?

NOTE: There appears to a trend wherein low thrust engines are preferred to be tested horizontally while high thrust engines are preferred to be tested vertically. Also, there are a couple of cases which is contrary to this rule-of-thumb. So, what are the key decision parameters?