I have been on trips where we chose each of these options. Kayak or solo canoe.

The Kayak drawbacks are: small hatches make loading/unloading gear at portages very slow. Gear neeeds to be in multiple small bundles for ease of loading which is less than ideal for portaging.   As for portaging the boat itself, I have seen and tried overhead rigs that allow you to portage with weight on both shoulders, but I thought they sucked.  The best way was one arm inside the cockpit, stretched toward the front with the cockpit combing on your shoulder. This can compete for space with a portage pack and also puts all the boat weight on one shoulder.

The solo canoe: much easier to load/unload and portage. Maybe a bit slower since you only have half the paddle, but This is now our prefered choice.

Neither the kayak nor the solo canoe could keep up with the tandem boats. But we didn't stress about it... we rotated crews and boats and just went with the flow.

We now use solo canoes whenever we have oddballs. We may even take two tandem boats and two solos on our 6-man trip this summer.

PJ in Hawaii