That's kind of hard to say without any specific requirements. I love jQuery and the abundance of plugins out there and also the ability to create your own plugins if you so desire, but the same could probably said for the Ajax Control Toolkit. It also depends on which one of these two you're more comfortable with. Do you have anything special in mind except a Calendar control as you implied?
As for the calendar stuff just try out what is there
AjaxControlToolkit Calendar[
^] and also have a look at the according
jQuery Calendar Plugins[
^] and take your pick of what you like best.
Best Regards,
—MRB