SSRIs (selective serotonin reuptake inhibitors) and SNRIs (serotonin norepinephrine reuptake inhibitors) are antidepressant medications which are prescribed for anxiety and depression. Both SSRIs and SNRIs are equally effective for anxiety and depression. For anxiety, SSRIs and SNRIs have a 75% response rate.
So when selecting an SSRI versus an SNRI, it is about side effect profile. Supposedly, SNRIs have less side effects than SSRIs, but this needs to be studied further in clinical trials. However, the SNRIs have more problems when trying to discontinue them, as withdrawal symptoms can appear, which is also called Discontinuation Syndrome. The withdrawal symptoms can include agitation, anxiety, paresthesias (pins and needles), shock-like sensations, sleep problems, headaches, nausea, diarrhea, tremors, and visual problems. And of all the SNRIs, Effexor (venlafaxine) has the worst withdrawal symptoms.
So although SSRIs and SNRIs have similar response rates, their side effect profiles differ, and that is what should determine which is prescribed.