The short answer is the Gran Sport is better than the Grand Prix; it's frame is constructed with higher quality tubing, and it was equipped with alloy wheels and crankset, making it several pounds lighter than the GP (this was mitigated a bit when the GP switched to alloy cranks in 1976). That being said, I have a '68 GP that I've retrofitted with alloy wheels and an alloy crank. That got the weight to within a couple pounds of a GS, and in my opinion it rides just as well. Couple this with the fact that you can find old GPs all day long for little money, and you've got a great starter bike.
In the '70s, Raleigh road bikes were built with 531 from the Super Course on up.