The OP specifically stated "tropics" but of course then followed up with 7mm of neoprene which would never be worn in the tropics.
If he is buying this for her for temperate water use, 7mm of neoprene, get a metal plate and the Oxy 30, if he truly is buying this for tropical use, 3mm neoprene full suit or LESS then the Oxy 18 with the Oxy Ultra Light fabric plate is the bomb.
The "stability" thing is way over rated. These small wings and fabric plates are designed for aluminum 80 tanks, which are what one will find for rental/guest use at nearly 100% of tropical destinations. There is NO instability until you start putting a lot of weight on the plate via a heavy steel tank or lot's of weight on the cambands. If you start putting a lot of weight in weight pockets on the cambands the rig will tend to roll a bit, why would you need to do this in the tropics? I use 3 to 6 pounds in my DR pockets wearing 3mm shorts and top, my wife uses 6 to 8 with her 1.5mm jump suit or 3mm shorty. How much weight you want to put on there?
Several, in fact most, new travel vests/jackets/back inflates use fabric or soft plates, the Oxy fabric plate works very similar to those, actually a bit more solid feeling. For light weight, travel friendly use, the Oxy 18 and Oxy Ultra Light plate are great, for heavy duty tech use with negative steel tanks, look into something else.
The real question here for the OP is travel friendly SP Equator vs AL Zuma vs Oxy 18 and Oxy Ultra Light plate. I would probably get her the Zuma.
Anyways, good luck to the OP and friend.
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