Doing the research into microphones was interesting for me. I actually started out doing sound recording on tape recorders before I got into photography, so I some experience with analog audio. With that background you might expect that I put emphasis on good sound. Actually, I suppose I would if I had the time, but I never do. Mostly I just use the camera sound as-is or I don't use it at all. In some cases, I wish I could re-do some of my videos and fix up the sound, but I don't have the time. That's why they are as bad as they are in the first place.
About the Sennheiser MKE440, from the YouTube clips that I found, I would say that the frequency response seems close to the Rode VideoMic Pro (mono). It seems to emphasize the mids and bass ranges, which is about the same as saying that it is weak in the highs. I did not run across any reports on self-noise, but I will assume it is quiet, because it's Sennheiser.
Videomaker gave their opinion that the stereo imaging was good to a relatively short distance. I can't remember, did they say around 3 - 12 feet? That sounds about right. From their test, I'd say that the peripheral sensitivity, falls off similar to a "wide" super-cardiod.
Overall, the performance reminds me of my JVC GS-TD1 (3D) video camcorder. This camcorder has a unique mic setup specifically designed to mimic "real human hearing" -- sound familiar? The mic spacing was wide and the response was a wide super-cardiod.What can I say about all this? Well, the mic sounds fine. I would not worry about it not being very "stereo" at far distances. The worst that happens is that the sound becomes more "mono". It is not going to sound bad, it just won't be very "stereo". Likewise the result in those cases will probably be "as good as" the Rode (mono). As a practical matter, I can tell you that a wide stereo image (on the other stereo mics) should not be a problem either.
What I would do on an analog mixer is put side on one channel and the other side on another channel and just cross fade them a bit into the middle. The result will be about the same.
So far, I have bought whole camcorders instead of mics. The Sony series all have stereo "zoom" mics. A "zoom" mic has a variable super-cardiod response pattern. On the Sony camcorders the response is linked to the zoom lens, so the sound sensitivity gets narrower as the lens zooms out. Even on my bottom end CX240 and CX405 it works, and in my opinion, it works really. Some people who have not used this system much say they don't like it. I think they might just need to use it enough to get accustomed to it. But if you get the better Sony camcorders (CX455 and the newer 4K AX53) you might not find any real advantage to separate shotgun mics. "Lavalier" mics or separate recorders are another story. You will probably still find those useful.
But going back to "good" sound being a matter of "taste". You seem to be young -- compared to me

, so this is what an "old man" can tell you: Your taste will probably change as you get older. In fact, it may change a few times. A couple of years from now, you might find that you don't like the sounds that you like today. So if you get a mic you like today, you might not like it in the future, and a mic you did not like today might become your favorite. It is a good idea to get a couple of mics with different characteristics. Use them and find out where one performs better than the other. If they are both "good" but different, you will probably find them both useful. Beyond that, don't worry about it too much.