Nero Express is the same as Nero. Same Engine, and same code.
But Nero Express is limited based on the Serial key used to install it. With Nero Express bundles, you only get the Nero Express interface, and can not go to the Nero Classic mode.
Those with original Nero Vision interface will always get this as it's based on the serial key you install Nero with. So if you have an OLD TDK bundle, you get the full Nero Classic option, and you can jump to the Nero Express GUI if you wish.
For those who purchase the new TDK bundles with Nero Express, they can not upgrade to the full Nero by a simple upgrade of software. They must purchase an upgrade from OEM Bundle to Retail for $19 US dollars from the www.nero.com