"12M New Wonder" is a misnomer.....they are two separate models, eh ?
There was New Wonder, then New Wonder II (the "Chu"), then Tranny (a transitional between the Chu and 12M) then the 12M.
I would say that, as with most Conns, it's the tone. They just have something to their tone which is unparalleled.
An 11M is a "Low A 12M". I believe they only started production in around 1961. These are good horns, they have been very misrepresented online as MX-made posers....but a usa-made one is quite good. Again, like the 12M...just massive sounding.
The other thing 'bout the 12M is...Mulligan played one...so, right there that is gonna set the popularity of the horn above all others.
I like 12M's, BUT...they are quirky beasts. I would actually argue a nice old King, Holton, Martin, or JK is actually a superior instrument as far as consistency and user-friendliness. But, I will get flamed for that.
They take time regarding intonation regulation...mouthpiece selection and such. In some instances, they require extensions to either the neck or m'piece shank. Then ....woodshedding.
Yet, despite all of these quirks, they remain arguably the most popular BigHorn. They are great horns. Like the 10M and 6M, it's because of the sound.
A contemporary model, in sonic comparison, can do nothing but cower in the corner....and I am talking ANY brand.
I would imagine you can find 12M recordings online. 11M, a bit tougher but they are there. If I had either up and running now, I would pop out a quick one for you but none of my current stock of 'em is playable at the moment.