Definitely a stereo pair of true omni mikes, whether it be binaural, OSS or spaced. The 635a is a great mike for a lot of instruments, and is cheap, so maybe that makes a good choice if you're worried you'll trash the mikes, but they do have a pretty serious low end roll off designed to tame wind noise (remember, it's an ENG mike, so it's optimized for recording interviews, not the rumble of thunder) so you won't get the full low end picture you would with an omni with flat response all the way down. At least with my RE-55s, the rolloff doesn't start until 40 Hz, and they are about as rugged as the 635a. If I was sheltered, I'd use the SM-80s, since they don't have any roll off in the audio band.
Cheers,
Otto