I'm not nearly as experienced a builder nor paddler as others that frequent this board, but I'll jump in with my novice's observations.

I recently built two Pygmys, a Coho (multichine) and Tern (hardchine).

On construction: the multichine took more time to construct simply because it had more bits to join into panels, and more panels to join to each other. On the hardchine, I had an extremely difficult hour or so trying to manhandle the panels into their correct positions to get them wired up, but I think its overall simplicity more than made up for that step.

On fit: the hardchine has more internal space out at the "edges" of the boat, which means I have much more foot room. (The foot pedal system has pedals only 4" or 5" wide, and at the edges rather than the center of the boat.) On the multichine, the hull curves inward quickly, which forces me to keep my size 10.5 feet canted, which sometimes puts some strain on my knees.

On stability: both boats feel quite tippy to a complete beginner (we've had friends try them). Once you're over that phase, however, the multichine is more stable in the angular region near vertical. In flat water it can feel very solid, in fact. The hardchine is more tippy in the initial region until you lean it over far enough to "hit the rail", and then it feels quite stable, more so than if you lean the multichine. In choppy water, I do not notice the initial instability of the hardchine whatsoever.

On maneuverability: I find the hardchine to be more responsive. The secondary "rail" stability is nice when doing a leaned turn... I don't feel like I'm going to topple.

On performance: supposedly the multichine hull is 3 or 4% more efficient (and hence faster) due to less wetted surface area. Ehhhh, big deal. Not an important consideration to me. I'm not racing anyone.

On looks: I like the lines of the hardchine, but that's just personal preference -- some of my family and friends like the multichine better.

And finally, overall, I like the hardchine better -- I'm willing to sacrifice the small amount of primary stability for the sake of maneuverability, I really like the higher secondary stability, and in my eyes it's just a prettier boat.

As always, Your Results May Vary.