Are they also interviewing external candidates? If yes, then I would dress in the stuffy pantsuit, because you'll be measured against them.
Are there a lot of other candidates for this position? If yes, then see above.
Do you know the hiring manager? If no, then see above. If yes, what is their style? ...dress accordingly.
Is this internal interview just a formality (you all but got the job already)? Then what you laid out is fine.
Another option is to wear a jacket (always a smart move), with pants or skirts (or over a dress) that compliment, but are not all matchy-matchy. This way you look pulled together, but in an effortless way. Plus jackets have the bonus of hiding any perspiration should you get nervous while interviewing.
Good luck!