Between the two, I'd say Promises is the best choice for her.
Minsky's needs a lot of work, and it's more of an ensemble piece, as it exists now. Too many main characters with not enough focused moments for each of them to shine (or to be fully developed).
Fran is a strong leading character, and Chenoweth could really shine in it. Great songs for her, too. I agree she's probably too old for this, but I think that's mostly fixable by casting an age-appropriate boss. If he is mid-to-late 60s, her "late-thirtyish" Fran would still be believable as his mistress.
Granted, this would be more believable with younger leads. It would bring out both the "cynically opportunistic" and "naively hopeful" qualities of the characters, as Billy Wilder intended. I think Sean and Kristin together will do very well, though.
"Jaws is the Citizen Kane of movies."
blocked: logan2, Diamonds3, Hamilton22