I find that the first stage and last stage give me just about the same pumpking / stamina rate. 10 stamina for roughly ~50 pumpkins vs 80 stamina for roughly ~400 pumpkins. I'd say they are about equal. Then it comes down to money and other rewards I suppose, but I'm out of stamina to make any more comparisons.
That said, I ran 7 event maps and triggered 0 sg's or bosses. Did anyone trigger a boss or secret garden during the event maps?