Object or UI Orientated?
Both libraries have a very similar set of features but I’d say the YUI is more mature than the beta ATLAS counterparts. The YUI components just seem to work as you’d want. When I launch a panel in the YUI I get the full dialog experience with options for title bars, modal dialog, move controls, etc. Whereas for ATLAS I seem to get a square that I can move, but I’ve got to add the mouse pointer, the title, etc, etc. Another example is the Auto-complete edit box. The YUI and ATLAS variants do pretty much the same job, however the YUI includes a "selected" event, whereas I seem to have to code that myself for ATLAS. This means that I can quickly produce pretty slick UIs with YUI that do 90% of what I want. However, here’s the bind. When the YUI component doesn’t work as intended it takes quite a bit of fishing around to understand how the component works to sucessfully alter its behaviour. Whereas with ATLAS you learn that up-front and changes are easier because it works as you programmed it to. Ok, that’s a bit unfair, with the ATLAS control kit you can pretty easily wrap core features to become something more like what you need out of the box. However, I still come back to thinking that the YUI did almost everything I wanted without any need to customise it.
With my short time spent using both sets of libraries I’d have to say I’d probably invest my time in ATLAS. Mainly because it seems like the logical path to take when developing Microsoft based solutions, especially when you take into account ATLAS’ XAML like script, surely the tools will eventually all come together. However, at this very moment in time I would say that YUI wins hands down for its easy of use and feature set. I thought I’d struggled to get around animation bugs in the YUI, well I’d not seen the ATLAS implementation!
I’m sure over time both libraries will improve and it will probably just come down to the sort of coding you need to do. If you’re a Microsoft shop I’m sure ATLAS or Microsoft AJAX Client/Server/Control libaries will be for you.