From what I've heard, most people when committing a crime don't evaluate the expected value of the harshness of punishment, merely whether there will be a substantial punishment at all. So long as you're attempting to use prison to deter crime, it argues for relatively light sentences. If you're trying to simply remove offenders from circulation, that's a different question. It won't really help them improve their character much though if the sentence is too harsh.

EDIT: A well publicised message of "if you do this we will catch you" and a well publicised conviction would help though mostly regardless of the length of sentence. Very few people find going to jail at all an acceptable outcome.