A leaked Samsung document allegedly shows off the expected time frame during which certain models will be receiving the Android 4.4 update. The devices in mentioned are the international version of the Samsung Galaxy S4 (GT-I9500), Samsung Galaxy Note 3 (SM-N900), Samsung Galaxy SIII (GT-I9300) and the Samsung Galaxy Note II (SM-N7100).
If this document is Accurate the Galaxy Note 3 & S4 will receive the Android 4.4 KitKat Update in January and the Galaxy SIII & Note II will receive it a bit later in March-April.
The new Android 4.4 KitKat improves the UI performance because of Project Svelte; the successor of Project Butter. This means all devices to be update with KitKat will become smoother and faster than ever, with even better multi-tasking and improved RAM management. We are sure Samsung will add somemore features to its TouchWiz UI, too.
At this point this is just a rumor, but we will keep you updated if the story progress further.