Ah, an either/or which straddles 2 forum categories! I saw your other post, but replied here as what I have to say is more about mics...
I think that if you want something which can be carried in a (largish) pocket, and is ready to go at the press of a button, you've already sold yourself on the D100. It seems to get a lot of love for the quality of its in-built mics, but I haven't used one and I don't know how quiet they are for truly low-level ambiences. It's an all-in-one solution, & there's much to be said for that. The only thing that I would add, is that effective wind protection for these devices can be hard to achieve. How important this is, is absolutely dependent on your local conditions.
The AT BP4025 is a great mic for quiet ambiences. The sensitivity is 25mV/Pa or -32dB @ 1Pa, (so about twice that of the Oktavas iirc) and it has very low noise (spec: 14dBA, rough measurements on my sample: 10-12dBA). AT has really worked on this design: the internal capsules are partially masked & set at 120 degrees, giving it a wide capture angle for all-round ambiences. What I really like about this mic is that the off axis response sounds smooth, and I think that's quite an achievement. The FR plot looks a bit scary, but in practice, when something moves behind the mic, it sounds good. I have recordings of street ambiences where people have walked directly behind me chatting, and the result is a natural sounding soundscape with none of the phasiness or strange colouration which you often get with directional mics (here's looking at you, NT4!)
But... (There's always a "but"). The central image is so-so, and the mic has no "reach" to speak of. I don't really think of it as a directional mic at all, as there's also no rear "null" as such. Although this is good in my example above, and for ambiences in general, it's not if you're recording street musicians and want to cut out noise from behind. In fact, that's one of the few instances where I'd choose the NT4.
There is another "But", though this is more about mic configurations in general, and that is that the coincident XY capsules will not sound as open as ORTF. But then you've used your Octavas in XY, so you'll know what to expect.
The BP4025 does just fit into a Rycote BBG, but it's easy to press the body too far into the ball so that the head just touches the internal mesh with the obvious consequences! Personally, I find the supplied foam plus additional fur sock (AT BPF-2) a more practical compromise and, with a gloved and steady hand, noise-free recordings can be made. The whole mic can then be stuffed in a jacket pocket when you're done. But I live on a very windy island, and invariably find that a full windshield kit is a better solution, for which my (original) Rode blimp gets pressed into service. So it becomes a fairly bulky solution, but less so than my spaced omni setup in two full windshields plus mounting bar & tripod...
The next option "up" would be MKH8040 ORTF in the Rycote stereo windshield. But, as you say, that's "Sennheiser territory"!