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My Barron's 501 Spanish Verbs book says for the two tenses:
- Serían las cinco cuando salieron.
- ¿Quién sería?
- Habrían sido las cinco cuando salieron.
- ¿Quién habría sido?
So what is the difference?
The difference is that when we conjecture about the past, we will have anchored ourselves in some period of time. For example:
Ayer serían las ocho cuando llegué a casa.
The time frame is yesterday/when I got home, and since we speculating about something concurrent/contemporaneous with that timeframe, we use the simple conditional.
If we establish a time frame in the past, but we want to speculate about something happening before that time frame (that is, it has already finished), then we use the conditional perfect:
Ayer cuando llegué a casa, mi familia habría cenado porque vi los platos sucios en la mesa.
Here we anchored the time frame in yesterday/when I got home, but our speculation is about what happened before when I got home, and thus being a completed (or perfect) action, we use conditional perfect.
Notice that this is the same difference between preterite/imperfect and pluperfect:
Ayer eran las ocho cuando llegué a casa.
Ayer cuando llegué a casa, mi familia había cenado porque vi los platos sucios en la mesa.1
1. This one would sound more natural with the first two clauses swapped and a few other minor elements changed (mi familia ya había cenado cuando llegué a casa, ya que…) but I kept the other order to show the parallel with the conditional perfect.
The difference is the normal conditional is used to express something while the action still impact in the present time while the conditional perfect is used to refer to something far in the past and the action has nothing to do in the present because it has finished.
For example: You are having lunch with some friends and there are some other friends who are getting late and you ask for the time when those guys left home. In that situation you should use normal conditional because the main action (your friends arriving) is still happening. In the other hand, if you ask again for the time when they arrived at the evening you should use the conditional perfect because they already arrived so the main action finished.
