123D is target at Makers and is intended to be simpler. Inventor Fusion is targeted at Mechanical Designers and includes more translators and functions for mechanical design. Over time the differences will be more obvious. They are based off the same technology today.
We are working to provide a complete solution to the navigation differences between Fusion/AutoCAD/Inventor and Alias. Stay Tuned!
123D is target at more general modeling for users entering into 3D and maker market. Inventor Fusion is a targeting mechanical prosumer users. They share technology but the differences will become more pronounced over time.