VT,
I'm not a Raleigh or tubing expert--others on the list know WAY more than I do.
That said, though . . . Reynolds 531 is better material than tensile steel.
But, as lots of folks on the list can tell you, handling and feel is more than just tubing, and lots of tensile steel frames have really nice ride qualities.
Other factors? Wheelbase, angles, and wheel weight. That last one's a biggie, too. The quickest way to change the way a bike handles, they say, is installing lighter wheels and tires. So you might want to look at the differences in the wheelsets of the respective Raleighs.
My guess (but I know nothing about the two frames, though I had a friend 30 years ago who had a Gran Prix) is that the Gran Prix has a tighter geometery, including a shorter wheelbase.
But . . . I'll let other, more knowlegable folks take over!
nath dresser spring green, wi (sunny here, but 3 degrees F at 8:00 AM. . . .)