>>1153475Welp eaither of those recorders will work nicely for that. Probably won't be worth buying external mics just because those preamps aren't great for ambient recordings.
The DR-100 with the omni mics will have a little better low frequency response and generally be more transparent sounding, but they won't record in particularly vivid stereo. They're also omni so you won't really be aiming them toward a certain direction, as they'll collect sound pressure arriving from every direction including backward around the body of the recorder itself.
The DR-40 will be a little "crisper" sounding and you'll be able to aim it in the general direction you want to record. The XY arrangement will be a fairly solid bur narrow stereo field and when you swing them out into the ORTF arrangement you'll get a very vivid and wide stereo pickup. This recording I got on my porch during a blizzard with the mics in the ORTF position. Was surprisingly nice. Also, condenser mics tend to be very sensitive and fail in high humidity or cold - these mics seem capable of tolerating almost anything!
https://clyp.it/psahjlz4 