Hi Alex,
I'm about 4 kg more than you and use the Custom 4 94 & 104 as my 2 board solution. I only do wave sailing here in West Australia and have clocked up > 200 sessions with those boards. For me it's a really tough choice between those 2 because of their respective strengths and limitations. Let's start with what they have in common: same design, very similar feel in gybes (ie a true wave board turn where you engage the rail and drive the board through the turn, weighting the nose on exit), very similar in the waves when sailed according to the conditions and with optimal setup. This mainly refers to the 104, where I needed to offset the rear strap and ensure everything else was optimal (mast, footstrap & fin position) to get the same bite on the bottom turn. Whereas the 94 was super playful and responsive regardless of that, the 104 felt stiff at 1st and really loosened up after I made those changes. Having done that, when I'm sailing the 104 in it's optimal wind range I hardly know that it's a 104, it feels so free and easy. The only caveat is that it sometimes feels as if it could nose dive when you hit a lip really late and drive straight back down into the pit. While it feels like that, it's never done it, so it might just be a perception coming off it's greater length. As you'd expect the 104 feels much more stable than the 94 in windless lulls, points really well (just like the older versions), but gets onto the plane much easier than the older generations. It can handle stronger gusts pretty well but when the wind really picks up it feels like a handful to me and there's never a doubt that you'd rather be on the smaller board.
The 94 does everything the 104 does, but in a more lively, squirty way. On the wave you have a bit more freedom, especially when the wind is up and the bigger board becomes a handful. While it is less stable at slogging, it's surprisingly good at this and because it also gets on the plane super quick it can also do really well on the lighter, variable wind days. For me choosing which board to use depends on the conditions and how busy it is. For example yesterday there were only 2 of us sailing the local break along with about 5 kiters. Waves were nice, up to mast high but the wind was very variable. I chose the 94 which turned out to be fine because initially when the wind was a bit stronger I could get on the plane and stay there with a bit of technical sailing. Coming in was all about slowly sailing high and then pumping onto the swell you select. Then you're rewarded with a super responsive board on a nice clean wave. If it had been busier I definitely would have chosen the 104 because I wouldn't have had the luxury of choosing whichever wave I wanted to ride. In busy times you need to stake your claim early and the 104 would have been better for this.
So as you can see it's a difficult choice between the 2 because they do different things. What about your C3 98? Could this be the 1 board compromise? Let us know how you get on.
Cheers, Jens