Citi Field (aka Shea2):
Good - Mama's of Corona. I prefer the turkey sandwich with gravy but most of the choices are good.

Bad - Kosher hot dog - took WAY too long to get cooked and served and wasn't all that great tasting
Ugly - Prices. It ain't cheap there at all.
Yankees Stadium:
Good - Sausages
Bad - Knish. Just bad. Soggy crust and not particularly tasty at all.
Ugly - Prices. Worse than Citi in most cases.