The 5.2 SL also has the Bontrager Race XXX-Lite fork, which is 90 grams lighter than the Race X-Lite fork that comes on the standard Madone 5.2.
Lately, I've been riding a standard 5.2, and for whatever reason, I find the grey/carbon paint scheme really boring. Personally, I'd look into the SL simply for the cool Midnight Duotone paint. It looks great in person.