Safer to banish them, at least temporarily, I thought. You were told Corypheus might still be able to influence them, and they'd lost a lot of members already. No point losing the rest of them as well.
This, pretty much, I agree with. The Wardens clearly need to be kept away from Cory. Unfortunately, you can't say "temporarily" in-game, which annoyed me. A problem with the writing, IMO.
Also, the choice doesn't matter. Not in the final battle--you fight Wardens anyhow, Cory jumps into a Warden, anyhow--and not for the story as it may continue post-DA:I. At the end of the game, either way.... well, it doesn't matter. It's clear Bioware are trying to either remove Warden influence from the game universe or set us up for heading north-west. Either way, it's all about Weisshaupt "going silent" (because it was so vocal to date *eyeroll*).
But to answer the OP:
However, the Inquisitor has to be careful, which may give reasons to pause and think. At the time of this quest, Quizzie doesn't know the dragon Cory has is not an Archdemon or that Cory uses Wardens to resurrect himself. They are, as far as anyone knows, the only ones who can kill an Archdemon (and if Alistair is The Warden, he says that outright to you).
Also, if you exile the Wardens, you are essentially exiling the Hero of Ferelden (Fereldens might take exception at their saviour being chucked out of his/her own country--especially if s/he is their Queen or Prince-Consort--also, Cullen's only alive because of the HoF. He might, also, take exception, and in most imported versions, you'd probably annoy your Spymaster, too) and, possibly, strictly speaking, the King of Ferelden as well if you've imported an Ali as King. You can make exceptions (you can for Blackwall), but in-game, you actually don't (except for Blackwall).
Also, again, at Adamant--many Wardens were missing--not all succumbed or obeyed (no Nathaniel, even if he's in an imported save, no Oghren--no dwarf Wardens at all, in fact--no Carver/Bethany, no anyone else from Amaranthine/Awakening) and so exiling them all for Clarel's error could be considered akin to Meredith punishing all mages for Anders'.
Also, you are told it was the Nightmare that created the fake Calling. Cory can influence if close enough (see Legacy) but not the whole lot, without help. You just castrated the Nightmare. No more false Calling.