I'm sorry to say this, but OpenCL (in particular, the implementation used for Adobe Premiere Pro) that's used for Radeon GPUs is still significantly slower than Adobe's implementation of CUDA that's used for GeForce cards.
And no, you cannot use both a Radeon card and a GeForce card together for GPU acceleration in Premiere Pro because Adobe has disabled OpenCL acceleration for GeForce and Quadro cards in all Windows versions of Premiere Pro that have been released to date (particularly in light of the GeForces' historically poor performance in OpenCL). Plus, using two completely different GPUs that are largely incompatible with one another in the same system is an invitation to severe stability problems.
Thus, if your PC is running Windows, getting the two different GPUs to run together for GPU acceleration in Premiere is completely impossible: Radeons cannot use CUDA at all while Adobe does not allow GeForces to use OpenCL in any Windows version of Premiere Pro. In other words, it's either, or or neither (the latter of which occurs when the Premiere Pro renderer is set to software only), but not both, in this case.
But that does not answer my question, only one small detail. The full question is (of the three) which is the better GPU configuration for rendering in Premiere? The 7970, 1050 OC, or Dual 660's by SLI?
I guess you have pretty much ruled out the 7970 due to extremely poor OpenCL, so at this point it's down to Dual SLI 660 and 1050 OC.
So at this point, the question is between the Dual 660 SLI and the 1050 OC for Premiere Rendering
Premiere does support two GPUs as long as they are not linked to one another. SLI can, and does, cause problems of its own in Premiere. So, if you do use two GTX 660s, ensure that they are not linked together via an SLI bridge; otherwise, the results will become unpredictable.
That said, if you take the two GPUs individually, a single GTX 660 will outperform a GTX 1050 in MPEG-2 DVD export rendering but will be slower than the 1050 in H.264 renders. This is because the GTX 660 has greater memory throughput but less efficient texturing units than the GTX 1050.
So then simply, 2 660's are better than one 1050. Thanks for the help!
To clarify about the "extremely poor" OpenCL performance:
It isn't that bad per se. It's just that if the 7870 we're to have been tested, it would have been as slow as or slower all around than even a single GTX 660, let alone dual GTX 660s. And even today, due to the deficiencies of Adobe Premiere's OpenCL implementation, AMD's most expensive (around $1,000) and fastest Radeon doesn't perform any faster than a GeForce GPU that costs $300 that's running in CUDA mode.