I have the first one here from NP (black with yellow upside down Y on the back), I guess it is unisex but I do not see any problem with that being a girl:
http://www.npsurf.com/products/protecti ... vests.html
It looks less padded than your preferred ones, but it works fine for me to keep my head out of the water while I sort out anything else I need to - I don't need to give any conscious effort to stay floating and can focus on whatever other problem I have - which I think is all the buoyancy you need, unless you are planning to cross the Atlantic or something like that.
While it was great when I bought it and I felt it also helped to keep the harness in place (it has a special surface at the waist), it has stretched now after a year (not more than 30 days on the water) and it does slide up a bit. So go for the smallest size you can fit I would say.
Looking at your choices, I think the front strap on the Serene will be totally in the way, I would stay clear of that one. The Spark looks like a good fit to me - it looks similar to mine, just more women-shaped. I would only be worried about the zip in the front - if the vest slides up, the zip might irritate your neck - mine has the zip on the side which does not cause any problem. Now after a few days of kiting, I have an irritation on my neck just from the hem of the lycra - I would not want the zip to be rubbing against my neck...