We have a later Mk II, and had the good fortune to tour a Mk I that came into the slip across from us a few weeks ago. I was quite surprised at how different the interior layout was. The Mk I was well maintained, but probably about 10 yrs older, so some of the difference might have been age related.
I'm not sure how to describe it, but the Mk I companionway steps are open, and sort of drop you down into an open area, with the quarterberth/nav station to port. On our Mk II the steps have bulkheads on either side, and you wind up standing with an open view of the galley/salon area. The Mk II quarterberth is private, with a door leading aft from the galley, while the head is to starboard. The Mk I has a bulkhead between the nav station and the main cabin which, to my eye, makes the main cabin look smaller though I'm not sure it really is. One big difference between our Mk II and the Mk I and most other Mk II's is the interior - ours is cherry rather than the standard teak, and seems to be quite a bit brighter and less cave like.
Engine access is probably slightly better on the Mk I than it is on the Mk II, though I haven't found it that difficult. Our Mk II has the 38 hp Westerbeke 38B Four rather than the 3 cylinder mentioned in the Sail magazine article.
One other difference is that ours has a sugar scoop stern (swim platform) rather than the traditional. Apparently that was an option on the Mk II's. Being baby boomers, it's slightly easier to board the dingy from the swim platform, though the freeboard is not so high that it makes a big difference.
The coachroof lines are also different, with the Mk I having two distinct handrails on two slightly different elevations, compared to our single handrail. Also the Mk I had the older style aluminum extrusion portlight frames whereas ours has the built in fixed portlights and heavy duty SS opening portlights (I don't really know, but those may have been an option).
From the looks of the sail dimensions, the Mk II mast is slightly forward - with longer boom and shorter J dimension, making jibs easier to handle. This seemed to be the trend with J boats as well, larger mains, smaller jibs.