I don't have any specific advice about either suit, but I can say that one of my kids has a mares 3mm and the other an aqualung 3mm and the aqualung does seem to be a superior suit. It's in subtle ways, but it wears slightly better, it seems easier to get into and out of, etc. Just small things, and nothing you can point to and say "clearly superior," rather it just seems better. If that makes sense. And of course, it could be differences in the kids as much as differences in the suits.
All that said, I live in a climate where 5mm and 7mm are the norms. We have 3mms for the two months in the summer when you can get away with it and for vacations. The rest of the time (and even in the summer if we're going to someplace deeper than about 20') we're doing the dry suits. They are simply more comfortable in those temperature ranges. I know they're expensive compared to 5 and 7 mms, but honestly, they are a much better buy over time. Just something to think about.
