The quietest mics are the Sennheiser RF mics, the 30/40 you mention. You can record quiet ambiences w/o the need for NR in post. They are not my favorite mics for capturing, but I like having them and they are reliable.
I also have a set of Schoeps cardioids for ORTF, NOS, Coincident etc. And a Figure-8 for MS. I like the sound I get with the Schoeps better, but I am not selling my Senn's.
That said, MS is my least favorite sounding recording, but it can be the easiest in terms of kit on a stand or pistol grip. I recorded motor boat sounds for a film using Schoeps MS and like having the rig be one hand-held Zeppelin. I could crawl all over the boat, hang off the side, explore the motor etc. VERY easily. I monitored AB as I encoded/decoded with the Sound Devices Pre and adjust the sides as I went for some great effects. For ambiences, I prefer ORTF, XY (coincident 90º), or spaced Omni's as a sound. I can ALWAYS hear the phase on the sides with MS. MS does give a stronger center (by as much as 6dB), so it is useful for many things, especially when you don't want wide. But it still sounds funny to me. Go to the Schoeps Showroom and flip through the patterns. It is fun and enlightening. The MS there is my least fav.
In terms of post : you will almost invariably have a problem with any downmix or pass through a Dolby Matrix with MS. Solution is to commit to XY (or, 2 channel decoded, hard-printed) when you are ready. You can MIX with the MS and play with the image if you want. But when you are ready to commit to a pre-dub or final, print the MS to a stereo track. I have done it with no problems and I have never wanted to go back and readjust the image, but you could if you plan your session right. I always leave the MS track in the session and then inactive and hide it once printed.
That's my tuppence.
Tom