A panel allows the user to work concurrently in it and in the main window, which is usually highly desirable (especially in cases like the source list). A sheet, contrariwise, blocks the main window until it is closed. That's often wrong, with things like the source list/editor definitely, in other cases it could be debatable; compare please the modality HIG.
Admittedly, there's some extra work involved, especially if the user works with more main windows at once (compare e.g., Finder's Inspector panel (File / ⌥Show Inspector). Nevertheless, given the app behaviour, it looks like you are most probably using the Cocoa Bindings for most GUIs, which make these tasks pretty easy and mostly “just-working”
Side note: the remaining sheets should be generally closeable by Esc (aside of the current ⌘w, as debated here; myself, I'd recommend to keep ⌘w and add esc as an alternative wherever reasonably possible without a clash with the current functionality).