KKj, I'd suggest you take some time to make yourself familiar with each framework. Depending on what kind of apps you like to create, I could well imagine that both are up for the task. IMHO qooxodo is the more comprehensive and mature framework, but of course being involved in qooxdoo I'm biased.
In general, creating desktop-style apps as well as web apps for smartphones are two requirements that aren't easily covered by a single framework. Usually there are tradeoffs in that you cannot just use all the features a framework offers for complex desktop apps now in a mobile environment. qooxdoo and Sproutcore have started to look into incorporating mobile-specific features, but I'd say for them (and others) there is still quite some way to go. Targeting fully featured desktop applications, you could start right away:
There is an (somewhat older) article about Popular JavaScript Framework Libraries: qooxdoo and SproutCore that you might find useful. qooxdoo is also at the Comparison of JavaScript frameworks at Wikipedia, Sproutcore unfortunately isn't. If you have some more specific questions, feel free to ask the qooxdoo community via the qooxdoo mailinglist. HTH.