In my hull cleaning business, I encounter water temps that range from 50° to 72°. December thru March I dive in a drysuit. The rest of the year I dive wet. I have my 7mm jons custom made and wear a variety of tops over it (depending on water temp), the thickest being 7mm and it keeps me pretty warm . I am quite comfortable with this much rubber on, although it does mean I have to load up with lead. But being as most of my work is within 10' of the surface, I am used to diving heavy.
In the transitional periods when it is starting to get cool or just starting to warm up, I'll wear my Mares Isotherm semi-dry. When new, it was virtually a drysuit. Now that it has a couple of seasons on it- not so much. But it is still warmer than the jon/jacket combo. This particular suit is really snug on me across the shoulders, so it's not the most comfortable suit I own. Otherwise I would wear it longer. Overall I like the jon because it's easy to put something thinner over it as the weather warms up. And, since I am in my suit 6 or 7 hours a day, I need to be able to strip down to just the jon between boats to stay comfortable. But if I were looking for a wetsuit for recreational diving and had your choice to make, I think I'd go with the semi-dry.