well lets say that 6xx 7xx 8xx 9xx is the line of products... in any line like 750 760 770 780 ... the one with the higher number will always be the strongest... Ti just a boost to the card... so 780ti is a 10% stronger 780...
card series doesnt always mean that each series have different GPU families... I can tell you that 750, 750ti, 970, 980 have a new "maxwell" processor that is very power efficient and well, brand new... so if you want a gpu, best go for that... the 760, 770, 780, 780ti and some 600 series is a "kepler" processor...
the 980 is almost as strong as 780ti (if you can overclock 980 to the wall then it will beat 780ti in gaming)
heres a chart for gpu performance