Brother KCS:
Hard to say without knowing more about the characteristics of the water body where you plan to paddle and how far you plan to go, but in general, a 12-foot boat is going to be slower than a longer one. With SOF boats, a couple of extra feet of length won't add much in weight. Of course, extra length can be an issue regarding storage. I'd say that either of the boats that you are thinking about will allow you to paddle a couple of miles in calm conditions (little wind or current) without too much exertion. Maybe you could borrow or rent a 12-foot plastic boat (there are many of that size) and try it out to get a better idea of what might suit your needs. Building a SOF boat will be fun in any case.
Fair winds, Andy