The sentence I want to translate is:
The wine cellar should be dark and dry.
Which of the following would be more appropriate and why?
La bodega debe ser oscura y seca.
La bodega debe estar oscura y seca.
In my opinion, it should be the first one, but actually, it could be both depending on the case.
I guess that wine cellars, in general, have to be
dry. So if you are talking about a general case it should be the first one. Let's put it this way:
All wine cellars should be dark and dry.
Las bodegas deberían ser oscuras y secas.
In the other hand, if this was not the case, and if you were talking about a specific moment for the
wine cellar (which in this case I do not think it would be often) it should be the seconde one. Let's change the example:
My house should be clean for this weekend.
Mi casa debería estar limpia para este fin de semana.
This is because you are talking about a specific moment (in time) of the house. It is not how houses are supposed to be, it is how they are in a certain period of time. If we change the example a bit again:
My house has to be bright and white.
Mi casa tiene que ser luminosa y blanca.
It is a state, not a specific moment.
P.S.: Note the gender of the adjectives in the examples,
bodega is femenine.
I have to say that, thinking deeply about this, this is what you will find most (like 90%) of times (for any example, not just this one). It is true that some people may use the second sentence as a translation for your question. I think that they would mean that
wine cellars have to be
dry as a moment in time, if you want to have good wine. But as I said, it should be the first one :)
The other answers point out an ambiguity in your original sentence: when should the cellar be dark and dry?
I'd rephrase the original question as
The wine cellar should always be dark and dry.
That translates cleanly to
La bodega siempre debe estar oscura y seca.
and I think it's a sentence you're more likely to actually hear when talking about wine cellars. Otherwise, if the original sentence belongs to a longer paragraph, I'd want to read the context to know how to better translate. Perhaps the text gives a general sense of the time. E.g., in instructions for building or cleaning a wine cellar, the "always" is implied. Finally, though it might be fine to say
La bodega debe ser seca
it's not a sentence that I'd ever write, because "dryness" is not an inherent quality of a cellar, but something that you have to make an effort to obtain.
Think of "ser" as a inherent property of the subject, and "estar" as a temporal property.
So if somebody "is handsome" can be "es guapo" or "está guapo", the first meaning he is a handsome person always and the second one that he looks handsome at that moment.
That's a good occasion to use the verb "permanecer". Otherwise, in spanish, "ser" is used as a way to describe the long-run state, while "estar" is used for short-run, varying properties. Similarly we use "estar" when we describe what we now do ("estoy pensando"), while we use "ser" for inherent properties and actions of the subject.