Hey Ed S,
I think that I have misinformed you at some point. You haven't ridden "the F-18" you've ridden "one version of the F-18". The WP F-18 comes in sizes from 4.0 to 6.0 feet, literally with increments of 1/4" so there would be something like 194 different length versions of that board. There isn't a standard size or configuration in the F-18
http://www.thewalkerproject.com/category_s/3.htm# Ryan,
The $300 price tag is going to limit you some. If you can find a used board, there may be more options. 5'6" is a decent size for you, but is probably longer than you need as an intermediate rider. Quite frankly at that price range and length you're looking at a WP Composite X 5oh or one of the big 3 offerings. The Composite X will be the fastest of the lot and is stable enough for teaching newbies. It doesn't have EVA traction, instead a version of Huladec...some folks don't care for the gritty surface.
For even a little bit more ($375 with eva traction) the Walzer Alpha comes in a 55" length.
Any chance you'd be willing to expand your budget?