The problem with the ferry is that it takes a lot longer. The ferry leaves Ludington at 8:30 am and arrives Manitowoc at 2:00 pm. That means it takes six and a half hours. That doesn't count the drive from Detroit to Ludington (three? hours). And, to make the ferry, you'd probably want to stay over in Ludington, so you'd kill Tuesday evening, too. More time... but a nice way to relax.
If you drive around the big lake, you could leave Detroit at 7:00-7:30 am, meet up with the Chicago convoy at noon, and start relaxing in Elkhart Lake by 2:30 pm. Or, you could leave Detroit around 10:30-11:00 am and go straight to the RealTime reception.
Decisions, decisions...
Incidentally, there may be folks coming from Montreal and Toronto who might be interested in hooking up with you when they get to Detroit.