Speed six = rounded butt and fixed sights. Barrel length usually 2-3/4" - 4", .38 Spl and .357 Magnum (and assorted other calibers.)
Security Six = square butt and adjustable sights. Barrel length usually 2-3/4" - 6", .38 Spl and .357 Magnum (and assorted other calibers.) Some very early models had fixed sights.
I believe I got it all. If not someone will be along shortly with additional information.