Unlike the mild hybrid, the two-mode uses a completely different and more complex transmission and larger battery. The hybrids also get a lot of weight saving changes such as aluminum hood and tailgate, lighter seats and wheels and a new brake system to allow for blending of regenerative and friction braking. It's estimated that the total package may add as much as $10,000 in cost to the vehicle and GM is contemplating eating some of that cost to make it more palatable to consumers.
The company will continue to offer multiple types of hybrid systems. The mild hybrid will be the value option, with the two-mode be the more efficient but more expensive option. In addition they may offer a variant of the mild hybrid with a lithium battery and possibly plug-in capability and their is also the previously announced two-mode plug-in system for the Vue. The E-Flex is not included in the hybrid lineup because GM considers it a range extended EV.
[Source: Automotive News - Sub. req'd]