[JasonB]

Another thing to consider, as if there weren't enough already, is what sort of layout you can fit in your space.

Scalextric track is not just smaller in it's width, but also in the radius of curves.

By way of comparison, both of these tracks would fit in a 20x5 ft space, but the Scalextric track is small enough to allow the switchbacks and nested corners, and is 70ft long. The Carrera track won't allow that and so it's 43ft long. I'm assuming you'll be adding borders, which is highly recommended.