From my perspective, vendor specific frameworks like the AJAX control toolkits only benefit from the ecosystems and examples published by that specific vender (and 3rd party devs). In this case, only Microsoft and ASP.NET developers.
In the case of a vendor agnostic toolkit like jquery, you gain the benefit from every type of web developer that's out there publishing blogs and articles: ruby, python, coldfusion, asp.net, etc.
So my vote is usually for jQuery unless there is a very specific reason (ie. specific control) that I absolutely have to use from the ajax control toolkit. And even in those cases, chances are that someone has a jquery plugin that'll be equivalent.