I was originally in favor of light tan, to match the interior and still like that choice, if only because I have a light greyish blue M.G. TF with a biscuit interior and light tan top. However, I am persuaded by Ireppond's argument for a gray top color that compliments, but is darker than, your Arabian gray.
Black is always a safe choice, because it goes with everything and doesn't show much dirt. Off-white and light tan (even medium tan) soil easily, so require much more care over time. A gray that is not too light will not soil so easily and provide the subtle contrast holding the entire color theme together that Ireppond suggests.
My Havana Brown 230SL has a Cream interior and Light Beige hardtop and hubcaps. However, the soft top is a brown that is slightly darker than the Havana Brown. It looks good with the hardtop, and looks good with the soft top, too! I have once saw a Havana Brown 230SL with Cream interior like mine with a tan soft top. I thought the car looked great but am glad I have brown as I wouldn't want to have to take care of another tan top.