If there wasn't much of a difference, they wouldn't make saddles in half inches, they'd make them in full inches.
It depends on your size and your personal preference as to what you're going to like. It also depends on the make of the saddle. I have a 45cm (17.5") dressage saddle that I love to bits even though technically it should be way too big for me - I have been "measured" as needing a 40cm seat size. Were I riding in a 17.5" Passier I would probably fall out the back of the saddle.
My advice is to sit in as many saddles as possible to find what you like. If at all possible, try out friend's saddles at the barn. Tack shops will also let you sit in their saddles, and some let you take them to the barn to try on your horse.