This is the most important difference between the 2. The Optoma's GT750E:
Decodes all HDMI 1.4a mandatory 3D format and SBS format commonly used by Blu-ray 3D players, game consoles and set-top boxes.
For example, if you have a PS3 you will be able to display 3D content without using an adapter.
The BenQ will require separate adapter to display 3D content from a PS2 or stand alone bluray player. The adapter can cost several hundred dollars, the Optoma 3D-XL costs $330 on Amazon or the Viewsonic VP3D1 $242.