We have a 6. From what I have seen there is no benefit to the 6 when doing short runs to the dock. A 2.5 moves a dinghy along at hull speed just as well as the 6.
It can get our dingy up on a plane for zipping around as long as there is only 1 adult and 1 kid in the dinghy.
I think it will be powerful enough to use as an emergency motor in case the main motor has a problem.
It is heavy and moving it from the X to the dinghy can be tough if there is a lot of motion. I always have the motor tied to the X when I am moving it so that if I were to drop it it will not be lost. Having it on a motor mount that drops down to the same level as the dinghy makes it much easier. Having a pulley system off an arch or swing out arm would eliminate the problem altogether.
So if you don’t want or need to plane and you are not worried about emergency propulsion then go with the 2.5. If planning or emergency propulsion matter to you then you need to weigh what is more important to you.