Besides sending Android 4.3, the update will make the phone compatible with the Samsung Galaxy Gear smartwatch. It also adds Samsung KNOX to the device. This offers a number of applications that are fenced in to prevent a security breach. Tapping the icon offers a secure browser, contacts list, email and calendar applications.
Considering the large size of the file at 781MB, your best bet is to make sure you have a Wi-Fi signal when downloading the update, and your battery should be at least half filled. These types of updates are usually sent out on a staggered basis, so if you don't get the update today, or tomorrow, it will come soon.
source: T-Mobile via AndroidCentral