At 78 kgs I believe you will prefer the 80 for the 5.0 and 5.3 days but may want the 74 for the stronger gero days. The choice would be for what days you would prefer to have the right size.
You are welcome to try my 80 kode to make up your own mind. I am 72 kgs and had the evo 70 as my main high wind board for the last few years and the evo 74/75 for the lighter days, for use at scabs, cott and mandurah. This year I tried the 80 kode for the jumping days at cott and for the lighter days as it gets going very fast compared with the evos and the ET.
At first I was a bit worried as it felt the same size as my 70 evo - that is it felt low on volume and could be used in the same wind as the 70. The 74 would be a straight swap for the 70 evo and the 80 gives a bit more "get up and go" for the light 5.3 plus days. My reaction was that I now had 2 boards for the same wind strength.
Having now sailed it heaps and on some good mandurah waves and plenty of scarbs days, I have found it is a very good comfortable board for full powered 5.0 days and 4.5 days locally, but in real wind in gero it might be a tad big. It is still very easy to turn on the wave and just as easy as my 70 evo but with a slightly different feel - more speed or traditional acid in feel but softer than previous years.
I could easily run the evo 70 kode 80 combination if I didn't like my 74 ETwin so much. I could have the 80 kode as a one board option for the majority of Perth days with no problems at my 72 kgs. It is only in the bigger winds up north that the smaller version would be more comfortable. Tye Bodycote recently gave up a 74 kode as he was too heavy for it at about 85 kgs and he knows how to get a board to work well!
So if you like your evo 70 but want something with a bit more squirt in between waves I think you may like the 80 kode.
Again you are welcome to try mine.