I'd have to say, for some people, the change of the drain from the cabin top to the side of the cabin is significant. Especially for those who are going to enclose the cockpit for use in bad weather, and those who leave their tug out uncovered. Leaving the tug out in the weather can result in a problem if the bilge pump is overcome by massive amounts of rain, or the scuppers get clogged. Another situation can occur when the weather is freezing, yet rain is still falling; the scuppers can freeze over causing the cockpit to take on water.
I've owned both Volvo and Yanmar diesels in several boats. To me they're a tossup. Both are excellent pieces of machinery.
The rest of the differences are superficial. They built a great little boat with small changes as they went along.