Excellent points, Pax. I was lazy and just chose the simple PDR answer! Yours was much better!
I'll go further. It's not a relevant question, really, because drugs aren't metabolized equally. Some of us metabolize certain drugs better than others, so thinking in terms of mathematical equivalency isn't useful. Which is to say, if Zoloft works for you and Lexapro just doesn't, what use is there is comparing dosages? Now, there are recommended dosages from the manufacturers of drugs, and ranges for those recommendations. The standard recommendation for Lexapro when I was on it was 10mg for depression and 20mg for anxiety, but for some, 5mg is plenty and for others 30 is just right. Metabolism is the real key here.
NO......... 15mg of Lexapro is STRONGER than 50mg of Zoloft.