I'd say she is stronger just for the fact that she is very much older. Also like another commenter stated, Krypton is a lot more dense than Earth so she very much should be stronger. I also read somewhere that her pod had some kind of solar power in it, but don't quote me. Another factor is that Supergirl is more ruthless than Superman so she doesn't hold as much back when it comes to fighting, but that still doesn't mean that he's stronger if she's older I think. Also, Supergirl is untrained where as Superman isn't. She doesn't get properly trained until Wonder Woman trains her so you have to think about that. She also absorbs the sun at a higher rate too, from what I know. But you also have to about the fact that Superman tends to hold back, but I still don't doubt that she can't potentially be stronger than him. You gotta look at this at a certain way, she could be and might be stronger, but we really don't know to be sure. Just think of it as a race. If they both fly at their fastest, they 9/10 will end at the same time. Thinking that one has perfect technique and the other doesn't. My bet is that the one without the perfect technique is faster. Just remember that there are many adaptations too.