Aside from the fact the Lumia 920 comes out in October....
Atrix HD
Runs current Android OS
HD Screen
8MP Camera
Minimal customizations from Motorola
Most likely will get update of OS from 4.0.3 to 4.1 (Jellybean) - after that it may have little to no support
Lumia 900
Runs current Windows Phone OS
Clearblack Display
8MP Camera
Minimal customizations from Nokia
Will get update to 7.8 OS but after that incremental security patches. After another 8 months may not be compatible with new apps
Google Play versus Windows Marketplace is moot. Either they have the apps you will use or they don't. iTunes might be the largest app marketplace but if people are just as content with what they get on their Android device they have no reason to switch.
Consider the following:
Are you buying the phone for what it does or what it will eventually be able to do?
Out of the box does the device do what you want it to?
Are you the type of person that has to have the latest tech?
If you answered yes to all three, I would wait. If you answered no to question three go ahead and get what works best for you. Keep in mind you are married to the device for two years on contract.