If you were using an app, and an error came up, would you like it to appear in a dialog or a sheet? 
Sheet, much cleaner look in my opinion.
Well, it depents.
Esthetically, I prefer sheets, they look nicer, are exactly where the App is.
HIG-wise, a dialog is a better choise, since an error something to alert a person, and thats what dialog-windows are for.
another aspect is, a sheet overlays your window, so if it would be useful to see the stuff in your window when you see the error (i.e. āyour input is wrong, please correctā), a sheet would not be a good choice.
so, itās a tough decision.
in the end itās up to you, i would prefer a dialog, since sheets are typically used to show progress of something, or to interupt the workflow to set up something.
a dialog on the other hand is that the application can tell the user something (hence ādialogā).
well, to be perfectly honest, thereās not much to discuss.
Every application on a mac should be HIG-conform.
this is the reason that mac-spftware feels ābetterā in general.
the spaces between the fields is standardized, the type of window used for tasks are the same, etc.
the HIG exists for a reason 
After reading, Iām sticking with a dialog.
Iām with Fabio - your exact use will depend on what the error is for. Is it a document-specific error, or an application-wide error. You must decide.
Chris