It is simple. If they are similar on all other aspects, and one of them wears jilbaab, then the one who is veiled adheres more to the islamic teachings and is, thus, "more religious". The question becomes problematic when they are not on par in all other aspects, which is often, if not always, the case.
People need to understand that those wearing jilbaab may not be better overall, but as far as the specific arena of concealing what Allah has ordered to be concealed is concerned, they are.
We should focus on our shortcomings and try to improve, and not try to belittle somebody´s efforts in one department because they lack in another. We will enter the grave alone.