Role
Perry: Kitty Purry will guest star as "Honey," a girl who may or may not date Ted and may or may not make Barney question his callous ways. She also may or may not have Katy Perry's, um, talents.Diddy: Per TV Guide, Diddy "plays a New York police detective named Reggie Williams who heads to Honolulu to hunt down the men responsible for a brutal crime against his family." Vigilante time!
Advantage: Diddy.
Bona fides
Perry: Katy Perry is hot, and thus was cast as a hot girl.Diddy: Diddy can play tough, as witnessed by his turn in Made. You can imagine him going off on some vigilante-fueled vendetta against his enemies.
Advantage: Diddy.
Importance to Story
Perry: The HIMYM producers say Perry will hold some key to Ted finding his wife. The key may or may not be in her cleavage.Diddy: If you've ever seen Hawaii Five-0 you know that guest stars have no bearing on the plot, or Scott Caan's peacock walk.
Advantage: Perry.
Audience Interest
Perry:HIMYM is watched by a fairly pop culture-attuned audience. Perry, love or hate her, speaks to that audience.Diddy:Hawaii Five-0 is watched by your parents.
Advantage: Perry.
Intangibles
Perry: Her cleavage.Diddy: His sunglasses.
Advantage: Perry.