Since the initial OP post mention about "without external power", and for Octane render.
Unless OP willing to go through the Pixlas mod. I suspect 2x970 actually work better than a single TitanX. Technically, 2x970 can safely draw as much as 300W from two PCIe slots and two 6pins. A single TitanX can only safely draw 225W. Even though we know that the stock TitanX (1000-1075MHz) 250W power draw is OK in the cMP, but that's technically exceed the official limit. And really right at the edge of the real world limit. If the card draw a bit more may activate the cMP's shutdown protection.
On the other hand, stock 970 (1050-1178MHz) only 148W TDP, just within the 150W limit. So, even without OC, 2x 970 has more CUDA core, higher clock speed, and most likely lower temperature. In fact, since everything still within the official limit, there is actually some room to OC the 970s and make them even faster.
So, IMO, even OP didn't ask for dual card setup in the original post. As long as OP have empty PCIe slots (need 2 slots for the 2nd 970). 2x 970 should be a better option under that specific condition. Two card not always work better, however, an extra slot do always able to provide an extra 75W, which can eventually turn into more performance. For the same Maxwell architecture, 300W power avail should provide better result than only 225W avail.