Hello, and thank you for the great question.
For a reduction in the the symptoms of depression, I recommend a very small dose of sativa or, even better, obtaining a vaporizer pen from hmbldt.com which was specifically formulated for depression; it can also be dosed precisely. http://hmbldt.com/pen-bliss/
There is preliminary research that has shown low doses of cannabis to activate serotonergic neurons, thereby increasing serotonin - and has even been shown to exert potent antidepressant-like properties in the rat forced-swim test (FST).
However, the same study showed that higher doses of cannabis actually reduced serotonergic action from baseline. http://www.jneurosci.org/content/27/43/11700
Considering the above, it becomes clear that cannabis is best used at the lowest dose and frequency possible to address your depression. There is both scientific and a plethora of adecdotal evidence that heavy cannabis use can lead to the worsening of depression. http://www.jneurosci.org/content/27/43/11700