There is a weight difference of about 10% (5kg) between the boats, and as most of the regular sedans can carry about 100kg on the roof there is no problem car toping a cat around 50 kg all included.
As to the construction, I think itís really up to what material you prefer wood or foam. As the cats are rather small there is no significant difference in building costs, the foam maybe a little bit more expensive. The time to build is about the same, also depending on previous experience with the materials. If you want to build a round bilged hull the foam is the solution, the tortured ply is asking a lot more hours of work for the same result, with the foam there is almost no filler and sanding and filler and sanding...
An other advantage with the foam is that you need very few and simple tools.
No test with kids yet if I dosen't count