7mm or 5/4mm wetsuit, sometimes with hood?

Shopping for a wetsuit for SCUBA and finally managed to find a reasonably priced "tall" size (I'm 6'2 and only 160lbs), now I'm having trouble deciding if I should go with the 7mm or the 5/4mm. Cost isn't a factor here because they're both the same and both are available in the medium-tall size I need. I only can't decide on the thickness because I want it to be versatile. I want a suit that can perform both where I live- Chicago area. And on more tropical/warm water trips. I originally planned (before seeing the 7 on sale) to get the 5/4 and use a hood for colder water. Now with the 7 being an option I know it would be better for my colder local dives, but am wondering if its overkill in, say, Hawaii or Puerto Rico. Any suggestions?