If you are going for a framework or a commercial AJAX IDE then first of all you should compare the different frameworks and IDE’s to make sure the availability of all the necessary widgets and functionality which would form a part of your application. There are a lot of frameworks which can help in your frequently used functionalities. For a start, you can choose from the immensely popular ones like DOJO, Prototype, Rico (which is based on Protoype), Script.aculo.us, etc
All the frameworks compete with each other in their implementation as well as the methodologies of request/ response. You might be less considerate on this aspect of technicalities but the thing which might be of more consideration is the number of rich features which they provide. The rich features comprise of the widgets like, an auto completing text box, a customizable calender, fade effects, an accordian , a dynamic menu etc which is most commonly required by a web application. The frameworks which I mentioned might provide some or the other combinations of these widgets. So for settling on a particular framework, you will have to work out all your features meant to be implemented in your application and only then finalize.
My own example
You must not combine a multitude of frameworks if you like features from many of them. All of the frameworks come with their own rich features as well with their own drawbacks. In order to finalize on a particular framework always compare and contrast the features of the frameworks so that you don’t have to jump to another one in the middle of your developmental efforts when you don’t find the current framework much of use.
Avoid the unnecessary pain that can be caused due to hasty decision of choosing a framework. You should not chose one just because the other guy told you that he found it cool. It might have been for his particular reasons but that isn’t the same for you always. Just choose one framework which best suits your needs. Its easier to maintain and use.