If you don't mind the size, you'll like the 920. On the other hand, the 820 is also worth a look, especially if you don't want a device larger than your 900. To date, neither my wife nor I have experienced any problems with our 920s, and I have to say that Windows Phone 8 is an excellent OS and successor to Windows Phone 7.
If you are in a situation where you're still under contract, don't need a new phone immediately or just are cautious regarding 1st generation devices, then waiting to at least see how WP7.8 performs us also a reasonable choice.