This is an issue that is the subject of some debate today, as can be seen in the New York Times link below. It can be argued in both ways.
From one perspective, it is important to allow judges to have some discretion. Human behaviors and human circumstances are so varied that it is impossible for any law to set out sentences that would be just in all situations. What this means is that judges need to be able to have some discretion. They see many more people who have committed crimes and have a much better idea than Congress does as to what sorts of punishments various convicts deserve. We should, then, let the experts have more latitude so that they can impose sentences that are just.
The problem is that this could end up giving judges excessive power and they could use that power to act in unjust and discriminatory ways. A judge might feel (consciously or not) that African Americans deserve harsher punishments than white Americans do, for example. In this sort of case, giving judges more discretion would make our system less just, not more.
If forced to pick one side, I would argue for more discretion, though perhaps with maximum sentences so that judges could not be excessively harsh. I would also require judges to specify their reasons for departing from sentencing guidelines.