Wellbutrin is in many ways a stimulant. It would generally be contra-indicated for anyone dealing with anxiety.
It can also be used as an adjunct anti-depressant, to counteract or augment properties of the main drug being used.
In psychopharmacology, what makes the difference between drugs is what is called the 'mechanism of action'. Classes of drugs (SSRI, SNRI, Mood Stabilizer, Anti-psychotic, etc) group similar targets of action - but the mechanisms of action are all unique.
It's trial and error. You decide based on whether or not the benefits outweigh the side effects, for you and your life.
Whether you want to switch to a new drug in the same class (an SSRI) or change class...there's no good way to decide. Your doc will have more experience with certain meds than others, that's usually how they decide which way to go.
If you want to be systemic about it, either add an adjunct (like a Wellbutrin), low-dose - or switch to a different SSRI, ideally one with as different a mechanism as possible. If it also doesn't do what you need, you can scratch off SSRIs from your list of options.
The only drawback of SNRIs is that they all have various degrees of discontinuation syndrome - they are harder to get off of, for many people, because of the unique headaches that arise. It doesn't happen to everyone. Cymbalta is the 'lightest' SNRI, so it's generally a good one to start with. It doesn't work long-term, but can be counted on for about 3-5 years.
Hope this helps.
