The 780 ti will normally be 8 to 12% faster in most games than a stock 970, but the 970 will use much less power and have more memory. Both cards can be overclocked, but the 780 ti can only gain about another 2% performance, while the 970 can gain up to 20%. This means that the 970 will only be about 2% to 6% slower if both cards are overclocked.
Even when overclocked, the 970 will use about 60 watts less energy than the 780 ti (while the 780 ti is not overclocked).
The 970 is based on newer architecture than the 780 ti and is much more power efficient.
That is my quick summary of the cards. Personally, I would always get a 970 over a 780 ti. Some people would do the reverse, it all comes down to what you prefer.