I will second that you should give Telerik a look. Their support is top notch in the industry. They give you good real world examples and their documention I feel is second to none.
I took on a contract with a company entrenched in the controls you're considering and I wouldn't use them if they were free.
The reasons I make this statement is their documentation in my opinion was bad. When I spend money on a RAD type control suite, it's to make my work easier and faster to production. I found in some cases it was easier to just figure out how to make built-in controls do what you want versus trying to figure out the problems I was having with their controls.
Their samples are kind of like Microsoft samples used to be. They are fluff based for Tech sales to show at a seminar "how easy something is to setup" but in real world if you used the techniques and monitored viewstate and traffic their examples generated, you'd be less than impressed.
I didn't have an account to submit support tickets but I had 4 over the course of a month submitted through the account holder and to my knowledge didn't get an answer back on any of them. (That could have been a break down with the person I had to go through but I doubt it.)
When it comes to Telerik's, Rad controls for Ajax, very seldom I can't figure out what I need to do by either looking at the sample Visual Studio sample web solution that get's installed combined with their documentation.
Good luck in your search and even if you don't consider Telerik, I would strongly suggest you look for other options.