Interesting story. It could be that you Dad was just used to the feel of the older model, but upon your testing of them both, it could be the newer bug was not as nice.
I only have one bug in my collection. It is a nearly mint condition Vibroplex Presentation, made in 1955, and was purchased by my father-in-law for my birthday a couple years ago at an auction. It had belonged to the last land-line operator for the railroad in my town, and when he retired, they retired CW, and he got to keep the key. After he passed, his estate was auctioned, and now the bug is alive again, and on the air, making QSOs. I would think that somewhere out there, that old landline operator would think it well that CW survived him, and that someone found his bug and put it to good use. Since I'm only 37, hopefully it will have a few good years left.
Anyway, I only have that one, and after spending a long time mastering it, I notice that I get pretty picky on how it is set. I've always heard that bug operators get really personal with their bugs and how they are set. Your post seems to confirm this as well. Thanks for the post. It was interesting to read. 73