Tandem canoes of such short length, in my experience, are just plain sluggish, so I'd go with the longer one. Even with the Mad River being just slightly longer, I think you'll notice the improvement. On the other hand, Pete may have a point about freeboard. Two big people, or two regular-size people plus camping gear may make you wish you had more freeboard, but whether that's the case or not, paddling an overloaded boat on a regular basis will sooner or later convince you to get something bigger, and then it will no longer be an issue.
I prefer traditional seats and really hate molded-plastic ones, but I'm a kneeler, not a sitter. My friend Rena recently got a tandem canoe and since we both kneel 99 percent of the time, we adjusted both seats for kneeling and it was SUCH an improvement. Besides the obvious boat-control issues and the advantage of all-day back comfort, kneeling eliminates the need for lots of legroom upfront, or the aforementioned problems of knees uncomfortably braced against the gunwales (which tends to become an issue in many boats when sitting, regardless of legroom, bow or stern, unless you install footbraces). I also fish from canoes and kneeling is a huge advantage for that as well. Unless you can't kneel due to some physical problem, I certainly recommend learning to paddle that way, even if just part of the time, and of course doing so requires your boat to have a traditional style of seat (in this case, Mad River again).
Bent Shaft Canoe Paddles
Full Size Sail Rig
Cartop Kayak Carriers
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