Microsoft has published a SKU Matrix for Windows Mobile 6.1, and there is also an older one for Windows Mobile 5.0.
Windows Mobile 6.1 makes it easier to stay connected and manage your busy life—from just about anywhere. Windows Mobile 6.1 is full of enhancements, made with your needs in mind. Think about this: When you text with a friend, the messages thread together, just like instant messaging—no more confusion about what you’re talking about. Connecting to a Wi-Fi network is now simpler—no more scrolling through multiple pages; a single click and you’re connected. And Windows Mobile 6.1 runs on a growing variety of phones, including touch screen devices with slide-out keyboards, sleek full-keyboard smartphones, and compact flip phones.