There are so many Ajax libraries, not even funny. Here is a good link about what is there.
Since we already used ASP.net, I took care to spend a little time getting to know asp.net ajax. This is enough to make the basics. However, for everything else, we use jQuery, which, by the way, goes well with ASP.Net. Have you ever wondered why the ASP.Net team started supporting jQuery: intellisense, included scripts, contributed to the project, made a demo with it, etc.
So here are my 2 cents. Learn the basics of asp.net ajax, but focus on IMHO, a better library like jQuery.
Good luck -D
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