If he gets the Ti and overclock a tiny bit, he gets the same as a titan black, which is WAY MORE expensive. Check this and compare by yourself (roll down the bar a bit for specifications):
http://www.hwcompare.com/17469/geforce-gtx-780-ti-vs-geforce-gtx-titan-black/
So I'd say hands down for the 780 Ti, don't hold back.