I really wonder what's the difference between one way signal and one way Path signal. I know they're not the same but my trains work with both of them. Also, building one way signal is slightly harder than building the path one.
One-way signals reserve a signal block between it and the next signal.
One-way path signals reserve an individual path between it and the next signal. This allows 2 trains to be on the same signal block at the same time, so long as they have paths that don't cross.
From the wiki:
The two new signal types behave a bit differently than standard OpenTTD signal types. The Path signals are red by default, and will only show green as soon as a train can reserve a path to the next safe waiting position on its route. Safe waiting positions are - by definition - in front of signals, depots and track ends. The back of a Path signal is not considered a safe waiting position, and therefore paths are reserved through these signals.
Because the front of every signal is defined as a safe waiting position, you would normally not want to place a signal immediately behind a junction, only in front of a junction. This is because it is only safe for a train to wait in front of a junction. It is not safe for a a train to wait at a signal immediately after a junction before the whole train has cleared the junction, as it would be blocking the junction while waiting, as illustrated in the example below. This is a major advantage against standard OpenTTD signals, where you had to place signals before and after junctions, which caused trains to block junctions while waiting.
For straight track and basic merges/splits, I stick to normal one-way signals. I use path signals for forks that have 4 or more tracks, as this is where they start to benefit.
Path signals also can behave similar to entry/exit signals, although entry/exit signals have uses that path can't fully achieve. For instance, entry/exit signals will cause trains to take longer routes if a path is blocked, while path signals may wait. This can be used to construct balancers and other helpful constructs.