I think citadel is worse than Ad Nauseam in the current dedicated Ad Nauseam storm decks for the reasons stated in the earlier posts (6 mana is higher than 5, bricks into lands etc)

The card is really powerful though and possibly becomes better than Ad Nauseam when
a) You can cheat it into play (e.g, in vintage you can slam this off tinker which seems ridiculous, but you already have combo options there with Jar/Vault and I'm not sure how much of an upgrade this is, if at all. It's also very good with Top in that format)
b) You've built your deck in a way that you're consistently using this to cast spells that cost 2 or more mana (e.g. it can be a potentially-castable Omniscience substitute in some kind of UB Show and Tell Lim Duls Vault combo deck, like a Dream Halls 2.0. It's arguably better than Omni by itself because it makes everything free but it also provides CA)