While only one has "mashed potato" in its name, both actually contain mashed potatoes as well as deli slices of turkey, and come on a pretzel bun. The Mashed Potato Burger comes with a beef patty and cheese, while the Spicy Chicken Burger comes with a breaded chicken filet.
They seem a bit overly sauced though. The Pretzel Turkey Mashed Potato Burger contains mayo, gravy, and honey mustard, while the Pretzel Turkey Spicy Chicken includes mayo, red paprika tartar sauce, and honey mustard. There's no cranberry in sight though.
Both burgers are available for 680 yen (~$5.69 US) for a limited time through January 25, 2015.