RadControls for ASP.NET Ajax are constantly improved and developed, the same is valid for the ASP.NET controls but in smaller scale. So, in order to compare both controls you need to choose a specific time frame for comparison.
Generally, you could examine the release
notes of RadControls for ASP.NET Ajax and the release notes of the latest official .NET Framework release
notes, additionally you could check the online help
documentation which demonstrates most of the features of our controls. The same could be done for the ASP.NET controls in the MSDN
the Telerik team
Consider using RadControls for ASP.NET AJAX (built on top of the ASP.NET AJAX framework) as a replacement for the Telerik ASP.NET Classic controls, See the product support lifecycle here.