Eventer, while it does sound to me 17.5 should be too big for you (I used 17.5 CC and I'm WAY taller than you and heavier too)
it really depends on how it fit your bum and the model/brand. My 18 dressage seems to be smaller than 17 A/P, for example.
Now for fitting I don't think anyone can tell if this saddle is better than that. You either want to talk to the saddle fitter or take pics with the saddle and post them for people to judge. OR talk to the tack store (like Trumbull Mountain Tack Shop - The Saddle Specialists
, they advise with saddles and they also have number of good saddles to choose from).
I think Circuit is nice looking and comfortable to sit in (I tried them in Dover before). However the problem with Circuit is because it's not a "brand" saddle (like Stubben or Pessoa or ...) it doesn't hold its value as good as those brands (at least that was my feeling when I did a search on Circuit while back).