More specifically I want to know the difference between 空（す）いていました and 空（そら）でした
For example, which one of these is better / what is the difference?
Is there an even better way?
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空いて【すいて】いる means the restaurant is not crowded (there are many empty tables), whereas 空【から】だ means the restaurant is completely empty (there are no customers). The same is true also when the subject is a theater, a train or a theme park.
Note that the correct reading of the latter is から. そらだ ("is the sky") is nonsense in this context.
You may also want to know がら空き【あき】(だ), which means there are very few customers.