もさることながら・はもとより: the difference
もさることながらN1
Meaning
Not only A but
Past papers
Tested 3 times in past papers
Example
モナ・リザは美しい画もさることながら、やはり目を引くのは神秘的な微笑である。
The Mona Lisa is a beautiful painting, of course, but what really catches the eye is that mysterious smile.
はもとよりN2
Meaning
Not only... but also, Let alone, Of course
Past papers
Tested once in past papers
Example
速く走るのはもとより経済的でもある車を買いたい。
Not only a fast one, but I want to buy a car that's also economical.
How to tell them apart
「もさることながら」: first grants A's value, then brings up a still better B; a refined, review-like tone.
「はもちろん」: general use, neutral 'A of course, and B too'.
「はもとより」: more formal than 「はもちろん」, same meaning: 'A goes without saying, and B too'.