I do not have both of these scents (at the moment). I adore Shalimar though and have heard many good things about the au legere (light) version.
I have heard that SL was created as a mirror image of Shalimar Classic with the emphasis been placed upon the citrussy top notes rather than the denser more intense powdery herbaceous vanilla base notes. So where Shalimar is very warm and creamy vanillary after a brief intense bergamot opening, Shalimar Light focuses and plays out the great citrussy start for most of the duration of the fragrance before settling to a clearer less oriental vanilla drydown.
I intend to get hold of a huge bottle of the Shalimar Light as it sounds a perfect way for me to be able to wear and enjoy the beauty of Shalimar all year round, especially in deepest summer when Shalimar can be come a bit cloying due to the oriental strength vanilla in the heat.
I do hope this has helped and I look forward to hearing what you think of the Shalimar Light! Enjoy!