Should these two be treated the same way?

My Cousland and Amell always spares Avernus.  Because it's like what Duncan says the Wardens would do anything to defeat the Blight.  I just make him do 'good' experiments.  As for Loghain, I had two Couslands who personally executed him.  I have one mage Amell that spared him for the same reason Duncan said about doing whatever is necessary.  Plus Loghain's a brilliant tactician and General.

I've only spared Loghain once as a mage, and I did spare Avernus at the same time.  But I specifically told Avernus to do good experiments that would benefit the Wardens.  Also Avernus also says when the Warden calls him a blood mage, he says that a Warden does whatever is necessary to stop the Blight.  So he echoes Duncan's and Alistair's words.  Alistair said Warden's would sometimes burn entire villages to stop the Darkspawn in their tracks.  So sparing both Avernus and Loghain is at the same time is small in comparison.  So yes, I've saved both in the same playthrough.