As most of the time we developers have a hard time choosing the right path. Discussions become nearly religious. But here to help to make up ur Ajax path, an aricle about Prototype and its relation to Atlas.

When you want to know more about Atlas i recommend the PDC05 sessions http://aspnet2.com/PDC05/Sessions.htm about this subject. They are working on cool stuff, the only downpart of it is, is that it isn´t going to come out in the nearby future. What i like about Atlas is that it wraps all the nasty Javascript coding by its serverside controls. Personally i dont like javascripting manually, to much time consuming without a descent debug tool. When ur short on time, just wait a bit with the Ajax stuff to let it settle down first.

http://www.dotnetslackers.com/Atlas/re-17513_Prototype_vs_Atlas.aspx