[Moved to hardware forum.]
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With the current version of Premiere Pro CC, CUDA still delivers superior performance to OpenCL. In fact, AMD GPUs at a given price point in OpenCL mode still perform slower in the current version of Premiere Pro CC than Nvidia GPUs that cost half as much as the aforementioned AMD GPU in CUDA mode. You may have to wait for the next major release of Premiere Pro CC (due out some time next year) for OpenCL performance to come anywhere close to catching up to CUDA performance.
Open CL still has allot of development work in the future regarding memory management that CUDA has already went through before it catches up in performance. Until then Cuda is going to perform better. The specs of the cards cant really make that up right now because they are still relatively close in performance. I would suggest a Nvidia card or you will be waiting a while before this changes.