They mean the same thing: “Roses are good.” The singular « une rose » still means roses in general.
The literal difference is more or less parallel to the English:
C’est bon, une rose. A rose is good.
C’est bon, les roses. Roses are good.
Notice how the English sounds a little awkward. A rose is good? It sounds like an unfortunate mix between having a particular rose in mind and talking about roses in general.
In order to save the meaning of the English sentence, you need to qualify the situation:
C’est bon, une rose, quand on veut être romantique.
A rose is good when you want to be romantic.
(Notice how this sentence would also be meaningful if you mean that a single rose would do.)
On the other hand, the plural is okay without any further context (though you certainly can provide some if needed).
I believe the French behaves similarly in this and similar sentences. Here’s another one for fun…
Un éléphant se conduit de façon irrationnelle. An elephant acts irrationally. (Which one?)
Un éléphant se conduit de façon irrationnelle en musth. An elephant acts irrationally in musth. (=all)