the 780ti has the full gk110 2880 cores available and running. the titan has 2688 cores running. the 780 has 2304 cores running.
the titan has different instruction sets available that the 780s dont.
a 780 can very easily be soft modded for some voltage tweaking and llc conditioning and get into the 1.4ghz+ range and will be noticeably faster than a stock 780ti. obviously the same can be done to a 780ti and it will beat the pants off of a 780. there are many links on how to unlock and fully tinker with a gk110. the best 780 to get is the evga classified, then the msi lightning and then the galaxy hall of fame.