Hi Chaim,
I've used Joomla bamboo's zend framework and there is a bit of a learning curve, however it's not steep, especially if you've created custom templates from scratch before. Although, in your case, the zend framework isn't the best choice as I don't think it's mobile compatible yet.
For T3 and Gantry, I've fiddled with both, and I would have to say that both have relatively the same learning curve. For some people it will be easier to learn T3 and for others Gantry. T3 is more feature rich, however perhaps not as "polished" as Gantry since Gantry has been around a bit longer.
Then there is also the construct framework which is relatively new to the scene and which I'm not too familiar with but has a lot of great potential:
joomlaengineering.com/
With that being said, your best bet is to test out the frameworks and find the one that works best for you. If you stick to one of the above, you won't make a wrong choice and it's only a question of making the best choice which is largely going to come down to preference rather than a clear cut winner.
Hope this helps!
Kind regards,
Nick