The Nokia 710 is likely to compete with the HTC Radar 4G on T-Mobile for the new Windows Phone user in the sub-$100 price range. Unfortunately, this means there is still no high end Windows Phone device available for T-Mobile while AT&T gets both the HTC Titan and Samsung Focus S. I have been using the HTC Radar 4G quite a bit lately and even though you can pick it up for free or $50 it honestly is a solid device and my only complaint is the rather disappointing camera.
The Nokia Lumia 710 is very similar to the Lumia 800, with the following differences:
The processor, operating system, apps, and display resolution are all the same. I find the HTC Radar 4G build quality to be fantastic and have to use the 710 a bit more before deciding which is better. I would love to see the Titan or Samsung Focus S on T-Mobile though.