I am with the other girls on here. My boy is a little over 16 HH and I am so tiny I need a 16.5 inch saddle. I do not think that it looks weird on him. As long as you fit and it fits your horse that is all that matters. Its about fit and comfort for you and your horse not other people opinions on what looks normal and what doesnt.
If its your saddle for life, go for which ever you feel more comfortable in.. but if you reckon there may be a chance you have to sell that saddle for whatever reason, 17" sells better ;)
I had a 16.5 and a 17 of the same saddle... 16.5 took FOREVER to sell, 17 sold in two weeks.. sounds daft, and I couldn't see the reasoning behind it, but when your general rider looks for a saddle, 17 fits most!
Also keep in mind different saddle makers will feel different from saddle to saddle. I just purchased a Dressage saddle after trying out a bunch - some 17.5 were just as comfortable as a 17, which is what I had always ridden in. I even ended up buying the 17.5 because honestly it seemed exactly the same as another brand in a 17.