For me, it's all about the air temperature and the number of repetitive dives. If it's 85 degrees on the surface and 74 at depth, I can probably wear a 3mm reasonably comfortably. If it's 65 degrees on the surface, or if I'm doing five dives a day or some number of longer decompression dives, I'll be miserably cold in a 3mm. Only you know your cold tolerances, but keep in mind your dive plans and the terrestrial forecast. Given what you said, I think you'll be more than warm enough with the 7mm wetsuit.
Personally, I don't own a 5mm or 7mm (just the 3mm and the drysuit), so I tend to want to go dry if the water is below about 75. I usually do 4+ hours of diving per day on trips, however.
Personally, I don't own a 5mm or 7mm (just the 3mm and the drysuit), so I tend to want to go dry if the water is below about 75. I usually do 4+ hours of diving per day on trips, however.