Talking about just the base reward rate (and not benefits like lounge accesses), magnus reward rate is good only when you spend 1L a month. There isn't much to gain if you spend a lot more than 1L and is kinda worthless if you spend less than 1L a month. That 1L is it's sweet spot. Similarly, the eterna has a sweet spot of something around 30-40k a month. Not much to gain after that.
So maybe both the cards aren't right for you.