Well, let's put it this way; 13.5 is usually just one heat consisting of national champions. 17.5 is usually 3-4 heats consisting of every skill level from Scooby on down to a first timer. The nice thing about Horsham is that we resort after every heat, so it only takes one run at your pace to be resorted with other people of similar skill level.
Bro, 13.5 at Horsham is practice for the national champs. Heck, even 17.5 is stacked these days! Scooby doesn't walk away in 17.5, which should be a good benchmark for you to judge which class to run.