You should be able to set your preferences to px in PS
Edit -> Preferences -> Units & Rulers -> Type (select pixels)
It also depends on the resolution of the file. If you work with 72 DPI files, it should be pretty close, at least, if you're using web-safe fonts.
It's really irrelevant - it will never render "exactly" the same. The anti - aliasing settings and rendering engine in Photoshop is independent of the operating system's type rendering in various browsers, so it will always be "just different enough" to make a visible mismatch. Pick one or the other and go with it. :-)