I love flat decks myself and really can't understand why some shapers put so much dome in. The Quads are also flat decked.
It's not super easy to comapare the Kode 74 and Quad 71. They are quite different. But I will try. The Kode is a modern version of a classic single fin. Really smooth and stable and with a quite fast rocker. The bottom shape make it really easy to get its rails in the water for long and stable turns and it have a rather direct feel where you can really feel the rails doing their job. The single fin design will make it a tad slower going into the turn and the board prefers when you commit into the turns. Top turns are loose and easy, but you can feel there is some power in the tail. In really fast conditions a smaller fins will help to keep the board loose. In more onshore type conditions, the sailor need to still keep some drive through the turns, since this is required for the board to turn tight.
In comparison, the Quad (71) is a tad wider and has a wider tail, but the quad setup still makes it quick in the turn entry. Not as quick as some twin fins, but much quicker than single fin of similar shape/width. The Quad also has a lot of drive in the bottom turn which the Kode also has, but the Quad is just one step above which for example means that in onshore, it's really easy to generate and keep speed when you need to draw your turn out a bit. But a difference to the Kode is that I fins the Quad 71 a bit easier to tighten up in the turn even when you are not perfectly balanced. In other words, the Quad is less demanding for doing tight bottom turns in onshore-type conditions.
In the top turn, the quad is again easy to get into the turn relative how wider it is, ie on par with the narrower Kode. It generates a quite powerful and grippy top turn, but it's also here a bit easier to tighten the turn up also to finish up further back towards where you can from.
So, as I see it there are lots of benefits with the Quad. But at the same time, the Kode is such a super smooth and dependable board. You just KNOW how it will react every time. The Quad may be a tad earlier planing (not much difference) and may go better upwind, but the Kode is still kind of faster from the get go and is also really, really fast when fully lit. Also in a straight line you can feel the predictability and dependability.
So overall I find the Quad offer a bit of novel feel and that there is maybe more wave performance to find in this shape. It do most things really well but then adds some stuff that other boards I tried just don't have in their repertoire. The Kode on the other hand is also a great allrounder. It the latest and most fine tuned version of the classic wave board as well as a really good high wind freerider. In some conditions it may require some more technique to equal the Quad, but in the other hand the smooth and predictable character of it is also very inspiring when it comes to charging harder.