The SPF has a steel roof, the CAV has a stainless steel monocoque chasis and is also sealed better from a water-entry perspective. Most of the differences are hashed out over at
www.gt40s.com.
Pat, just my opinion, but I like the shape of the SPF better; however, I would not be adverse to owning a CAV at all. It just would be my 2nd choice between the two offerings.