Hi Michael ,
To be frank with you, it would be pretty difficult to compare the frameworks, not only gantry there are many other frameworks available for Joomla templates offered by many clubs. Each come with its own advantages / disadvantages and features. Some are good for developers and some are easy for end users.
It is not possible to use two framework with one template. To make this point clear, i would like to remind you that, framework basically holds the features and templates run on them, each framework has its own specific / unqiue classes whicch are required for the template to render. T3 classes are different and so are of Gantry.. the template will not be compatible and will not render in the absence of the classes. But yes, you can have template assignments for different menu items to use the frameworks, but i would not see any point in overloading any joomla site with too many css / js files which comes with the framework.
Try them out, find the one that suits your requirements.
Arvind