The look: Brooklyn pours a bit darker, with a little less head. The Sam Adams is quite a bit cloudier than Brooklyn. 
The smell: Brooklyn smells like hops, Sam smells sweet. Despite them both being called summer ales, they are completely different styles, as Brooklyn is a pale ale, and Sam Adams is a wheat beer.
The first sip: Brooklyn is surprisingly mild, with a hint of sweetness, very different from the smell. Sam is more like what you'd expect from a summer beer. Very light without a hint of lemon, great to drink freezing cold on a hot day.
The aftertaste: Despite their differences, both follow in the summer style in that they are refreshing with very little aftertaste.
The decision: Brooklyn has more flavor, but I would say Sam Adams is more refreshing, so if you plan on drinking inside, go with Brooklyn. If you are drinking outside, go with Sam Adams. You can't go wrong either way.