Which of the orientation is preferred for static fire testing of a liquid rocket engine - vertical or horizontal?

Question

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?

Answer 1

Vertical orientation allows you to test the engine and its plumbing in a configuration that resembles the launch configuration.
A horizontal test stand requires a fuel and oxidizer supply that works differently to the ones in your rocket.

Generally, horizontal test stands are used during engine development. They're cheaper to build than vertical stands, but due to the above there are some inaccuracies you have to account for.

The FH test fire you linked to was a test for the entire rocket. The engines in that rocket may have been tested individually (when each engine is produced) and before that, there were many development tests.

Safety-wise, I can't see a big difference. In both cases you need foundations that can withstand hundreds of tons of thrust, and in both cases you have a horizontal flame trench.