The GTX 970 is roughly on par the same performance as the GTX 780 but runs at a lower TDP, comes with new awesome features and is more future proof with driver optimizations on the way. I recommend getting the GTX 970 card over the 780 unless the GTX 780 price drops enough where it might be better to get the GTX 780 only because price/performance for it.
I rather go for the GTX 970 only because of newer features and the potential graphics card driver optimiztions made for the GTX 900 series meaning it may outperform the GTX 780 later on and you also get 4GB GDDR5 VRAM vs 3GB which is good if you plan on doing multiple displays or 4K resolution in the future. Also, the newer technologies such as DSR, VXGI and MFAA included in the newer card are nice to have. Enjoy your new card!