All our controls now have classic and ajax editions. In other words, RadComboBox.Net2 is the dll of the classic combobox edition, whereas Telerik.Web.UI contains all our controls, including RadComboBox, that represent our latest generation controls built entirely on top of MS Ajax. If you are going to build your application from scratch I would suggest that you use the ajax edition of the combobox (as part of Telerik.Web.UI) as this edition provides lots of improvements. For more details, please refer to the following link:
What's new in RadControls for ASP.NET Ajax
Having any other questions, please feel free to ask.
the Telerik team