When using a computer for media such as photo and video editing, the amount of RAM in the computer makes a big difference. That is the amount of fast accessible memory in which files are stored for immediate access as you are working on them. When the RAM is small, the computer has to often clean up for more working space and this has a speed impact. When talking about CPU clock speed, the difference between 3.4 and 3.5 GHz is really insignificant when they are the same model of CPU (Intel i5 in this case). The one may have slightly larger cache memory or something like that, but I do not believe the impact will be nearly as significant as a 100% increase in RAM.
I would therefore recommend that you rather get the 3.4 GHz with a 16 GB RAM upgrade.
The difference between CPU and GPU is this: CPU stands for Central Processing Unit and it is this chip that runs the system and controls all aspects of computation in a general way. GPU stands for Graphics Processing Unit and its job is to render the output shown on your screen, frame by frame. It is incredibly sophisticated in dealing with lots and lots and lots of complex math calculations (Billions per second). Usually, this is associated with gaming and a faster GPU with a lot of memory will render more detailed, smoother imagery in a 3D computer game. But, operating systems has become more sophisticated and most software these days make use of the GPU’s incredible and specialised mathematics engine to assist in rendering math-intensive things like video etc. So, in your case, in addition to the CPU and RAM, a good GPU would have a positive impact on performance while using Premiere Pro or Final Cut.
I hope this explains a few things :-)