It's considered obvious now, after the CRIME attack was published, but 15 years ago everyone would have said that you obviously should compress first! (I remember this being asked as a homework question when I was in college, with "compress first" being the intended answer.)
Indeed, I think the conventional wisdom was that compressing first would in fact improve security a little bit. This idea goes back all the way to Shannon in 1949, who noted that compressing before encrypting should improve information-theoretic security, because if the messages contain redundancy, then an adversary with infinite computing power can use that to decode the message. (Just try all possible keys, and see if it decrypts to an actual English sentence.) On the other hand, if you first compress using an ideal compressor, then every compressed plaintext will look the same (just random noise), so every possible encryption key will produce some plausible plaintext.
Indeed, I think the conventional wisdom was that compressing first would in fact improve security a little bit. This idea goes back all the way to Shannon in 1949, who noted that compressing before encrypting should improve information-theoretic security, because if the messages contain redundancy, then an adversary with infinite computing power can use that to decode the message. (Just try all possible keys, and see if it decrypts to an actual English sentence.) On the other hand, if you first compress using an ideal compressor, then every compressed plaintext will look the same (just random noise), so every possible encryption key will produce some plausible plaintext.
[1] http://netlab.cs.ucla.edu/wiki/files/shannon1949.pdf ; see sections 16 - 19.