Tough decision.
If you are using this primarily for work purposes the app usage is going to remain mostly focussed on stock apps like Notes, Tasks, Calendar, Contacts I would assume and not require App Store content.
Windows mobile has a few quirks - I enjoy the email support and if work email is all exchange based I'd certainly consider the Lumia 920 for support on that, Add Nokia DRIVE software if you use your phone in the car and need SatNav. I find it better than Google Maps navigation.
However windows mobile people / contacts has a quirk that irritates me in the fact that the font displaying the users contact number and details is very small and you can't really make it much bigger in the phones settings either making quickly grabbing a persons contact number details a little frustrating and unintuitive.
Samsung Touchwiz offers a nice calendar app for keeping your appointments and meetings and its easier to read on the fly (if not a little cluttered looking) than the one in Windows phone 8, and Contacts app (whilst I don't like Touchwiz visually) is easier to use / display contact data than on windows mobile too.
Add the fact you can add widgets to calendar events and shortcuts much easier n Android and you could possibly have more of your work information easily viewable / accessible on the Samsung S3 than the Lumia 920.
Windows live tiles don't offer the same level of information on quick glance that androids widgets can offer.
So yeah,.. I'd probably opt for the S3 if I had to choose one, and it was for my work only.
