oh sorry, I completely forgot that they are both the same price.
Even though the 780 ti has the advantage in pure horsepower, the gtx 970 counters with more Vram, way lower TDP, and a better feature set.
So, here are some questions you have to ask yourself:
1. What games do I play. this is important because the gtx 780 ti only has 3GB of RAM, and that has proven not to be sufficient for ULTRA settings in AAA tiltles like battlefield 4. So your basically gona have stuttering, which is just as bad as frame loss, where as you won't have this issue with the gtx 970 because of it's higher vram count.
2. Do I need/want the extra features that Maxwell cards bring?
3. (not as important) can my power supply handle the GTX 780 tis wattage, if not then get the gtx 970. If you have a 600W or higher PSU, then this question is obsolete.