An integrated graphics chip (iGPU) uses a portion of main system memory to store the data it needs to do its calculations. A discrete graphics chip or card (dGPU) contains its own memory, or Video RAM, which is generally a lot faster than main system RAM.
As main system RAM (eg DDR3) is quite slow compared to normal VRAM (eg. GDDR5) iGPUs are generally slow, as anything faster would be bottlenecked by memory bandwidth. As dGPUs don't have this problem, they are generally faster. Much, much faster, and use a lot more power.
On the other hand, the bandwidth between an iGPU and the CPU will be higher than that between CPU and dGPU, so some simple calculations will perform better on an iGPU, increasing the benefits of having both iGPU and dGPU in the one computer.
EDIT: A 670M or 680M in the iMac will be far faster than the 6750M in your MBP. My guess would be around 3 times as fast. If you do not require portability, there is no reason to get a MBP over an iMac. If you do require portability, the iMac is not an option.