Jeremy, I know the topic isn't the type of canoe you want to buy, but I'm curious whether this is your first canoe or an additional one. I ask because, to me, an 18.5' tandem canoe with no rocker is a niche canoe. It's primary purpose would be to carry very large loads on very long trips on big water. It will be at the bottom of the list of maneuverable canoes and would be a beast to paddle solo.
Most people can take average trips just fine in a 17' tandem or even a high volume 16' tandem such as a Mad River Explorer. These shorter tandems will be more maneuverable, more amenable to solo paddling, and lighter in weight.
Of course, if you already have a 16'-17' tandem and are specifically are looking for something bigger, then a Minnesota II or an old MR Lamoille may make size sense.