The C&J is definitely not as interesting. But in the long run it will never go in, or out, of style. It also will be more versatile in terms of wearing it with jeans/chinos, especially as it ages.
As much as love the sleekness of the C&J 348 I find it somewhat too much on my two pairs of Merton loafers. And I wonder if in a loafer I wouldn't be happier with a slightly rounder last. The Simpson last on the Carmina's is not quite as long as the 348. If they offered this makeup on the Rain last (which is a little wider) you would be in 'sweet spot' IMO.
Then we have the issue of the suede itself. Carmina is known for it's luscious suede. C&J is just OK as far as I'm concerned.
Of course the easy answer is, BOTH!
I prefer the Carmina as well although I haven't tried both pairs - the C&J in in a G fitting so slightly wider than usual. My feet are on the borderline of medium and wide so I'm not sure whether a narrower last like the Simpson is the best idea. If they offered this on the Rain last, it would be a no brainer. Then again, suede stretches quite a bit the more it's worn so maybe getting a pair that's snug and letting them stretch might be the way to go.
I nearly got the Merton some time back but decided it didn't suit my foot for the reasons you state above.