There isn't much difference between the 2 as they are < 50 yards apart and share shuttle service to BWI. If you are a platinum, you are much more likely to be upgraded to a suite at the Sheraton.

The bar/restaruant is much better at the Westin in my opinion. Wider selection of beer & wine and an inventive menu. dining at the sheraton was ho-hum.

also, the breakfast spread in the club room is much better at the Westin.