"Good to hear" vs "Glad to hear"

Solution 1:

In terms of their conversational meaning, they are completely synonymous. Use whichever one you feel like.

There is a very subtle difference in meaning, in that the first expands to It's good to hear that you enjoyed the radio show, while the second expands to something like I'm glad to hear that you enjoyed the radio show. Because of this, the second is slightly more personal, and the first slightly more general.

This difference basically never matters in conversation.

Solution 2:

There is no real pragmatic difference;
both are appropriate, both are grammatical, and both express the same sentiment.

There is a very small semantic difference,
between personal and general satisfaction.

However, there is a very large syntactic difference between the two expressions.
Both of them are examples of Conversational Deletion, which deletes expectable initial words.
In this case, the two expressions come from very different sentences:

The first one

• Good to hear you enjoyed the radio show

comes from the sentence

• (It's) good to hear (that) you enjoyed the radio show.
  via that-Deletion, and Conversational Deletion, which deletes the predictable
  contraction of a dummy it subject and auxiliary for a predicate adjective: (It's) good.

This sentence comes in turn from the sentence

• To hear that you enjoyed the radio show is good.
  via Extraposition, which licenses the dummy it that gets deleted in conversation.

Note that this sentence doesn't say who it's good for;
the experiencer of goodness is Indef.

The other expression

• Glad to hear you enjoyed the radio show.

comes from the sentence

• (I'm) glad to hear (that) you enjoyed the radio show.
  via that-Deletion and Conversational Deletion, just like the first one;
  what gets deleted is a contracted subject and auxiliary for a predicate adjective: (I'm) glad.

This looks similar to the first one, but note that it does say who is glad;
the experiencer of gladness is identified as the speaker, not just Indef.

Solution 3:

Either is correct English but "glad" is more personal. You're suggesting you personally are happy to hear that they enjoyed the show, whereas "good to hear" just means it's good [for someone] to hear.

Edit - or, @choster put it so nice and succinctly, "I am glad to hear," "It is good to hear."