Must be stereo. And goal is to capture outdoor ambient sound, not anyone speaking on camera.

Why do you think you need stereo shotguns for this?

Rode Stereomic X.

Gary, the Rode is a really good mic, but is not shotgun and picks ups any handling of the camcorder as I move around, this was the first mic I tested. I need something more directional that also reflects any handling by the user.

I am torn between:

Sony ECM-680S M/S Stereo Shotgun Microphone $850
and the
Sennheiser MKH418S - M/S Stereo Shotgun $1650

Both of these require phantom power, so an external preamp will need to be added as well.
Is the Sennheiser worth double?

Anyone else have a suggestion for a pro setup? This is for outdoor nature type footage, where the camera might be moved to catch action, so I need mics pointing same as camcorder all the time. Can't use off-camera sound crew situation.

I use the Rode Stereomic Pro w/ the AX100 w/ pretty good results. As w/ any camera-mounted mic, a good shock mount is imperative. I had a Sony 680S that I'd occasionally mount on a EX1R. It is an excellent mic - probably overkill to go for the Sennheiser for your purpose - but it is rather long, and if not getting into your shot will likely mess w/ your balance, as you'll also need a Beachtek or equivalent. You might consider a more isolated shock mount with the Rode.

I have a Sony ECM678, sort of like the mono version of the 680. I can tell you it is extremely long.

If you want AMBIENT sound, don't get a shotgun. They do pinpoint isolation well, but they don't have the audio clarity of less directional microphones. Perhaps look into those little stereo 3.5mm jack microphones that are made to be used with mini audio recorders, like for DSLR's?

Paul