i've owned both, and both are great, but the melissas are a lot more deluxe. if she loves wakeboarding and will use them a lot then shell out the extra cash for sure.
my only reservation is that it's really important to try the melissas on first.
i think i'm a size 7 too (as far as i can figure the difference between australian and US shoe sizes) - i used to have medium minx which were fine. I bought melissas in size 7-9, and they fitted ok, but i have a fair bit of heel movement which annoys me heaps! i've since tried on the smaller pair and they're perfect for me. I think i just have really narrow feet for their size, so even though i technically bought the right size, they weren't the best for me because i didn't take into account my foot shape!! complicated i know. they're just such a molded boot that you really need to make sure you're getting the right size. it's not worth spending that much money if they don't fit properly. i'm now trying to sell mine so i can buy the smaller pair (5-7)