Well, of course, if you're going to throw Logic into the equation, you would be getting more shaders/CUDA cores for the money with the pair of GTX 970 cards. It also allows me to get one card now and the second one later to ease the finance. Plus, it sounds like there might be new GPUs being announced in the next 6 months which might drive the price of the second card down.
Of course, on the other hand the memory bus is only 256 bits instead of 384, and we're talking 1664 CUDA cores versus 2816 cores. AnandTech Benchmarks for the 970 vs. 980 Ti shows about what you'd expect: The 50% more cores and memory yields improvements in most benchmarks of half again the performance, but some metrics are equal or actually worse for the 980 Ti.
Logic. Thanks for being a buzz-kill.