I don't know what you mean by "essentially the same", but there are some significant differences. There are reasons you might doubt that San Francisco exists, the primary two of which are: 1) you believe that only things described by fundamental physics are real, everything else we talk about is either reducible to physics, or some sort of strictly inaccurate approximation to some physical reality. 2) You don't believe (1), but still think that for some reason San Francisco seems too vague of an object.

But that said, if San Francisco does exist, it exists in space and time. It didn't exist until the past 500 years, though the land it inhabits existed before then. It's also between 1 and 13 thousand miles from the Easter coast of China. You can locate it, you can go to it, etc.

None of those things are true of the number 5 (http://plato.stanford.edu/entries/abstract-objects/).

It's perfectly open for someone to deny that either or both of numbers and San Francisco exists, but the considerations seem a little different.