Our pediatrician actually preferred for me not to be on an SSRI while breastfeeding unless absolutely necessary because it does excrete into the breast milk. When I asked my OB/GYN, he deferred me to the pediatrician who rendered his opinion.
I would ask a pharmacist to do a comparison for you - there are SEVERAL medications in the drug class, some may have done studies to have FDA approved use for lactation, others may not have sought that approval, but the manufacturer may have data to support its use or not.
I'd also ask the pediatrician to see what they recommend as well as your OB/GYN.
Without getting too scientific, there's a BIG difference between using a product during pregnancy vs. lactation and different kinds of scientific studies are required by the FDA to be approved for each.
As far as using during pregnancy, usually, they want to make sure that a product doesn't cross the placental wall and have an effect on the developing baby. Whereas, many things can be excreted through breast milk - that's why you have to watch your caffeine consumption, spicy foods, etc. Each product will have either received FDA approval or not for both different categories.
The world of medicine is always changing. By law, the FDA is required to collect and keep all safety (including side effect) data on file. If a product is shown to be unsafe after being on the market a while, the FDA will restrict its use, issue warnings where it should not be used or pull it from the market.
Feel free to send me a message if you want me to help you further - I've been in the pharmaceutical industry a LONG time and may be able to clarify a little better.