I've been doing this exact same thing Onkel... for work though. I've been capturing various atmos (Parks, beaches, suburban streets, city streets, children playing etc) for the post production audio guys at work.
At the moment I am just taking a laptop with Mbox mini out with me. I am using a pair of Rode NT5 set up in ORTF configuration.
I would stay away from XY for recording spatial sounds like atmos. XY is suitable for close mic'ing stuff like guitars etc, but for the nice wide stereo stuff M/S, ORTF, or spaced Omni's seems better.
So the Zoom option would probably be better, due to the fact you can use other mics.
I am planning at some stage to get a Fostex FR2-LE:
One thing you should do if you are using studio mics for outdoor recordings is get some decent wind socks for your mics. Let us know how you go...